Some strange shipout behaviors (plain tex)

I was trying to understanding how the size of a page is determined in terms of TeX primitives. As first attempt, I’ve tried to occupy completely the page with a simple header (a line) , a footer (in form of a big T) and centralized text content. But I’ve failed in this mission ("You lose!"). Please, take a look in my code.

 % initial configuration
voffset=-1in
hoffset=-1in
hsize=10em
vsize=10em

% defining layout page (at least, trying it)
output{%
    shipoutvbox{% this layout was uncaple to eliminate all margins! Bottom and right margins persist
    leftskip=0pt
    rightskip=0pt
    hrule height 0pt depth 7pt
    vfill
    box255
    vfil
    hrule height 7pt depth 0pt
    hbox to hsize{hfilvrule height 70pt hfil}}}

% hyphining at will (or almost this)
pretolerance=45
tolerance=10000

% local defintions
defcorrection#1{%
            ifcase#1
                moveright
          or moveright-
          or moveleft
          or moveleft-
            or kern % this generates a disaster! It's reasonable, but I haven't got it
            or kern-
            else
                    defeliminatehsize##1{}
                    expandaftereliminatehsizefi}

defarg{4} % in this cases, leads to kern

% finally, text content of page
vfil

noindent Umas verdades quar seram tambem vadjust{correctionarghsizevbox to 0pt{hbox to 0pt {vbox{hsize=7emnoindent Mulambo Silvestre}}}} semper que tardia nostrum vitae moscaribatus.
vfil
penalty-10000 
end

It could looks ungly at first but it’s really simple. So… questions are:

1) Why there are a right and a bottom margin in the output? (I’m usingbTeX 3.14159265 (TeX Live 2019/dev/Debian) and displaying dvi files with evince)

2) Why the box255 was not centralized?

3) Why a weird output (creating new pages!!!) when arg values 4 (a kern is used as correction). I hope it was lost only part of the content, the same as in arg = 5 or arg = 1.

Thanks in advance.