Changing arrows to dashed line in smartdiagram

Adding dashed in the arrow style gives your first requirement

                additions={
                additional arrow style=dashed,
                additional arrow color=black,   
                additional arrow line width=2pt,
                          }

EDIT -- for the labels

I found it easier to apply TiKz which is a superset of smartdiagram -- deleting about 15 lines of smartdiagramconnect and additions and adding the decorations library the dashed arrows and labels are easily positioned

Using

        -stealth,
        line width=1mm,
        red!40,
        dashed,

the arrows are defined

With the help of decoration

                    postaction={decorate,
                    decoration={text along path,
                                text align=center,
                                text={Diatessaron},
                                raise=5pt}}

the labels are positioned and raised 5pt

With the help of

(module6.north east) to[out = 45, in = 85, looseness = 1.2] (module5.north 
west);

the anchors are provided to the corner of the module and the arrow angle of departure and arrival are added

With the help of xshift anchor position can be shifted to the left or right

([xshift=.5cm]module5.north) to[out = 90, in = 180, looseness = 1.2] 
(module4.west);

Complete MWE

documentclass{article}
usepackage{smartdiagram}
usesmartdiagramlibrary{additions} 
    usetikzlibrary{arrows,quotes,calc,decorations.text,positioning} 
begin{document}
    begin{figure}[ht]
        centering
        smartdiagramset{uniform color list=teal!60 for 7 items,
            module x sep=3.75,
            back arrow distance=0.75,
            sequence item border color=black,
            uniform arrow color=true,
            arrow color=gray!50!black,
            circular distance=5cm,
            font=large,
            text width=2.5cm,
            module minimum width=2.5cm,
            module minimum height=1.5cm,
            arrow line width=2.5pt,
            arrow tip=to,
        } 
        smartdiagramadd[circular diagram:clockwise]{
            1-Define the construct,2-Identify and describe facets,3-Determine facet levels,4-Determine scenario structure,5-Develop mapping sentences and build scenarios,6-Select response options and survey instructions,7-Test (and retest)
        }
        {}
        begin{tikzpicture}[overlay]
        draw[-stealth,
                line width=1mm,
                red!40,
                dashed,
                postaction={decorate,
                            decoration={text along path,
                                        text align=center,
                                        text={Diatessaron},
                                        raise=5pt}}] 
        (module7.south east) to[out = -45, in = 85, looseness = 1.2] (module6.north);
        
        draw[-stealth,
                line width=1mm,
                red!40,
                dashed,
                postaction={decorate,
                            decoration={text along path,
                                        text align=center,
                                        text={Diatessaron},
                                        raise=5pt}}] 
        (module6.north east) to[out = 45, in = 85, looseness = 1.2] (module5.north west);
        
        draw[-stealth,
                line width=1mm,
                red!40,
                dashed,
                postaction={decorate,
                            decoration={text along path,
                                        text align=center,
                                        text={Diatessaron},
                                        raise=5pt}}] 
        ([xshift=.5cm]module5.north) to[out = 90, in = 180, looseness = 1.2] (module4.west);
        end{tikzpicture}  
    end{figure}
end{document}