How to prevent vertical stretching of blank space?

I am using reledmac to prepare the edition of a series of texts. Each text starts on a new page, and is preceded by 2 lines of information (manuscript shelfmark and folio), in a description environment. I need the 2 lines and the title of each text to appear exactly at the same place on the page, for each text.
Most of the times it works well, but in some cases reledmac seems to have trouble calculating the page break at a convenient place, and "stretches" the space between the 2 lines of description at the top of the page.

In the example below, the blank spaces between the 2 lines of description (MS1 and MS2) and between the description and the beginning of the text are overstretched in the first text (Sermo secundus), and normal in the second text (Sermo tertius).
My example is a bit long, sorry for that, but I cannot use lipsum because I need to show the actual text with critical footnotes which create the issue, in a given layout.

documentclass[,twoside]{article}
usepackage[oldstyle,proportional]{libertine}
usepackage{fontspec,xunicode,microtype,polyglossia}

setmainfont{New Aster LT Std}

newcommand{changefont}{%
 fontsize{7.5}{9.74}selectfont
}

usepackage[fontsize=10.5]{scrextend} 
  
renewcommand{footnotesize}{fontsize{8.5}{11}selectfont}
 
usepackage{setspace}
setstretch{0.92}
 
renewcommand{footnoterule}{
 kern -3pt 
 hrule width 2cm 
 kern 2pt} 
 
usepackage[paperwidth=170mm,paperheight=240mm, headsep=8pt, twoside, bindingoffset=0cm, inner=2.3cm, outer=3.5cm, top=2.5cm, bottom=3.0cm]{geometry}

 
 usepackage{fancyhdr} 
 fancyfoot{}
 renewcommand{headrulewidth}{0pt} % remove the header 
 pagestyle{fancy}


 fancyhead[LE,RO]{thepage}
 fancyhead[CE]{changefont SERMONS} 
 fancyhead[CO]{changefont EDITION}
 setmainlanguage{latin}
 setotherlanguage{english}
 usepackage[series={A,B,C},noend,noeledsec,noledgroup]{reledmac} 
 Xarrangement[A]{paragraph}
 Xarrangement[B]{paragraph}
 Xarrangement[C]{paragraph}

 Xbeforenotes[A]{18pt}
 Xbeforenotes[B]{18pt}
 Xbeforenotes[C]{18pt}
 Xlemmadisablefontselection[C] 
 Xnotenumfont{normalfontbfseries} 
 lineation{page} 

 setlength{stanzaindentbase}{20pt}
 setstanzaindents{1,1}
 setcounter{stanzaindentsrepetition}{1}
 
 firstlinenum{0}
 linenumincrement{5}
 linenummargin{outer}
 Xnumberonlyfirstinline[] 
 Xnumberonlyfirstintwolines[]
 Xsymlinenum{}
 fnpos{critical-familiar}
 
 
 makeatletter
 newcommand{TEIsection}[1]{vspace{0.5cm}noindentbegin{center}uppercase{#1}end{center}vspace{0.4cm}}parnobreakvspace{-parskip}@afterheadingnoindent
 makeatother


 
 
 
 begin{document} 

begin{description}
item[A] MS1, f. 1v–2r
item[B] MS2, f. 3rb–4vb
end{description}

 beginnumbering
 
  pstart[vspace{1cm}]
 TEIsection{Sermo secundus}
 pend[vspace{0.5cm}] 
 pstart edtext{emph{Sanctificabor in hiis qui appropinquant michi} (Leui. X).}{lemma{}Afootnote[nosep]{Lv. 10, 3.}}
 pend
 
 
 pstart Narrata hystoria, nota duo competentia ordinandis~: dignitatem officii cum dicit~: emph{in hiis qui appropinquant michi}~; sanctitatem ministerii~: emph{sanctificabor}, id est sanctus apparebo. Primum in suscipiendo ordines~; secundum in exequendo.
 pend
 
 pstart Quantum ad primum, notatur dignitas que magna est cum apropinquatur ad Dominum. edtext{Num. XVI~: emph{Sanctos edtext{applicabit}{lemma{applicabit} Cfootnote{applicabunt emph{M}}} sibi et quos elegerit Dominus appropinquabunt ei}.}{lemma{}Afootnote[nosep]{Nm. 16, 5.}} emph{Elegerit} scilicet ad dignitatem ad quam non elegit angelos.
 pend
 
 pstart Debent autem appropinquare ad Deum in dolore contritionis, in timore purificationis, in feruore dilectionis.
 pend
 
 pstart De primo, edtext{Iac. IIII~: emph{Apropinquate Deo, et apropinquabit uobis. Emundate manus, peccatores}, per satisfactionem, emph{purificate corda}, per confessionem, emph{miseri estote et lugete}}{lemma{}Afootnote[nosep]{Iac. 4, 8-9.}}, per contritionem.
 pend
 
 pstart De secundo, edtext{Deut. edtext{XXXIII}{lemma{xxxiii} Cfootnote{xxxii emph{P}}}~: emph{Omnes sancti in manu illius sunt, et qui apropinquant pedibus eius, accipient de doctrina illius}.}{lemma{}Afootnote[nosep]{Dt. 33, 3.}} Cuiusmodi sunt illi qui ex timore se ad pedes Dei humiliant et indignos se reputant, sicut sancti qui promotiones antiquitus fugiebant.
 pend
 
 pstart De tertio, edtext{Iere. XXX~: emph{Quis est iste qui applicet cor suum ut apropinquet michi~? ait Dominus}.}{lemma{}Afootnote[nosep]{Ier. 30, 21.}} Hii sunt qui a terrenis cor suum separant ut se Deo uniant, et ex dilectione quam habent ad Deum et proximum se ad ordines suscipiendos preparant.
 pend
 
 pstart Sed quidam econtrario appropinquant non in dolore sed in exultatione, non edtext{in}{lemma{in} Cfootnote{emph{om.} emph{M}}} timore sed in presumptione, non in dilectione sed in ambitione.
 pend
 
 
 pstart De primo, edtext{Amos VI~: emph{Ve qui opulenti estis in Syon}}{lemma{}Afootnote[nosep]{Am. 6, 1.}}, et sequitur~: edtext{emph{qui apropinquatis solio iniquitatis, qui dormitis in lectis eburneis et lasciuitis in stratis uestris, qui comeditis agnos de grege,  lednopb qui canitis ad uocem psalterii}}{lemma{}Afootnote[nosep]{Am. 6, 3-5.}}, ubi nota triplicem exultationem edtext{inordinatam}{lemma{inordinatam} Cfootnote{inordinatis emph{M}}} in actu, gestu et auditu.
 pend
 
 pstart De secundo, edtext{Iosue III~: emph{Cauete ne apropinquetis ad archam}}{lemma{}Afootnote[nosep]{Ios. 3, 4.}}, propter quod edtext{II Reg. edtext{V}{lemma{V} Cfootnote{emph{rectius VI.} }} percussus est edtext{Oza}{lemma{oza} Cfootnote{ossa emph{M}}} a Domino, qui indignus accessit ad archam}{lemma{}Afootnote[nosep]{Cf. II Rg. 6, 1-11.}}, in figura eorum qui edtext{indigne}{lemma{indigne} Cfootnote{indignus accessit emph{ante corr. M}}} ministrant uel edtext{promouentur}{lemma{promouentur} Cfootnote{monentur emph{P}}} nec seruant continentiam. edtext{Exo. XXVIII~: emph{Utentur Aaron et filii eius} femoralibus lineis emph{quando apropinquant ad altare ut ministrent ne iniquitatis rei moriantur}.}{lemma{}Afootnote[nosep]{Ex. 28, 43.}}
 pend
  
newpage
 
endnumbering
 
begin{description}
item[A] MS1, f. 2r–3v
item[B] MS2, f. 4vb–6ravb
end{description}

 beginnumbering

 pstart[vspace{1cm}]
 TEIsection{Sermo tertius}
 pend[vspace{0.5cm}] 
 pstart edtext{emph{Sic nos existimet homo} etc., usque ibi~: emph{ut edtext{fidelis quis}{lemma{fidelis quis} Cfootnote{quid fidelis emph{M}}} inueniatur} (I ad Cor. IIII).}{lemma{}Afootnote[nosep]{I Cor. 4, 1-2.}}
 pend
 
 pstart Informat Apostolus promouendos in tribus, edtext{scilicet in sanctificatione}{lemma{scilicet in sanctificatione} Cfootnote{in sanctitate emph{M}}} que pertinet ad meritum, in ydoneitate que pertinet ad exequtionis officium, in fidelitate que pertinet ad premium. De primo, edtext{emph{ministros Christi} }{lemma{}Afootnote[nosep]{suite de I Cor. 4, 1.}} id est uncti edtext{gratia sanctitatis}{lemma{gratia sanctitatis} Cfootnote{emph{om.} emph{P}}}. edtext{Psal.~: emph{Oleo sancto meo unxi eum}.}{lemma{}Afootnote[nosep]{Ps. 88, 21.}} edtext{Eccli. X~: emph{Secundum iudicem populi sic et edtext{minister}{lemma{minister} Cfootnote{ministri emph{M}}} eius}.}{lemma{}Afootnote[nosep]{Eccli. 10, 2.}} De secundo edtext{emph{dispensatores} }{lemma{}Afootnote[nosep]{suite de I Cor. 4, 1~: emph{et dispensatores mysteriorum Dei}…}} etc. De tertio edtext{emph{hic iam queritur ut fidelis edtext{quis inueniatur}{lemma{quis inueniatur} Cfootnote{emph{om.} emph{M}}}} etc.}{lemma{}Afootnote[nosep]{début de I Cor. 4, 2~: emph{Hic iam queritur inter dispensatores}…}}
 pend
 
 pstart Sanctitas ergo promouendi notatur cum dicitur emph{ministros Christi}. edtext{Gal. II~: emph{Nunquid Christus peccati minister est~? Absit.} }{lemma{}Afootnote[nosep]{Gal. 2, 17.}} Et quoniam in Christo reperitur mundicia que declarabatur in incarnatione, sapientia que declarabatur in passione, potentia que edtext{declarabatur}{lemma{declarabatur} Cfootnote{declarabitur emph{M}}} in resurrectione, iusticia que edtext{declarabitur}{lemma{declarabitur} Cfootnote{declarabatur emph{M}}} in finali retributione, ideo promouendus minister Christi debet esse mundus, discretus, robustus et iustus, que pertinent ad exercitium IIII uirtutum cardinalium.
 pend
 
 pstart Debet ergo minister Christi esse mundus quantum ad continentiam. edtext{edtext{I}{lemma{i} Cfootnote{iii emph{M}}} Reg. II~: emph{Samuel ministrabat ante faciem Domini, puer accinctus ephot lineo, et tunicam paruam faciebat ei mater sua}.}{lemma{}Afootnote[nosep]{I Rg. 2, 18-19.}} Ephot lineo uestis linea carnis est mundicia, quam debet habere promouendus ut sit continens quantum ad affectum, quantum ad aspectum, quantum ad tactum, edtext{et}{lemma{et} Cfootnote{emph{om.} emph{M}}} quantum ad actum. edtext{emph{Et tunicam paruam faciebat ei mater sua}}{lemma{}Afootnote[nosep]{I Rg. 2, 19.}}, quia carnis integritatem eis qui promouentur in sacris indicit Ecclesia. edtext{I Reg. XXI~: emph{Si mundi sunt pueri, maxime a mulieribus} }{lemma{}Afootnote[nosep]{I Rg. 21, 4.}} manducent, quia edtext{I Thi. III emph{Probentur primum, et sic ministrent, nullum crimen habentes}.}{lemma{}Afootnote[nosep]{I Tim. 3, 10.}} Probentur quantum ad uitam, conscientiam et famam, uel probentur quia sicut episcopi debent esse probatoria clericorum, ita clerici promoti probatoria laicorum. emph{Et sic ministrent} obseruata continentia ad quam tenentur propter honestatem clericalis officii, necessitatem uoti, auctoritatem diuini precepti, dignitatem sacramenti. emph{Nullum crimen habentes}, quia edtext{Leui. XXI qui aliqua macula inquinantur ad ministerium altaris accedere prohibentur.}{lemma{}Afootnote[nosep]{Cf. Lv. 21, 17.}}
 pend
 

newpage

endnumbering

end{document}

First text:

Second text:

How can I make sure that the 2 lines of description and the beginning of the sermon are always at the same place on the page (vertically), and prevent the blank space from being stretched?
I tried using ledpb and lednopb in the sermon text, but to no avail. Alternatively, I tried using a table for the 2 lines at the top, instead of the description environment. I managed to prevent the stretch between the 2 lines, but then the space between those lines and the beginning of the sermon was stretched, it only displaced the problem.