> I cannot see a difference with forward-line.  I did this:
...
 > 5. In buffer a with point at (point-max), repeatedly typing C-p goes
 > like this, with `^' marking successive positions of the cursor:
 > line1line2line3line4line5line6
 > ^    ^         ^         ^
 > line7
 > ^
 >
 > 6. In buffer b with point at (point-max), repeatedly typing C-p goes
 > like this:
 > line1line2line3line4line5line6
 > ^    ^   ^     ^    ^    ^
 > line7
 > ^
 >
 > 7. In both buffer a and buffer b with point at (point-max), repeatedly
 > typing 'M-: (forward-line -1)' goes like in 6:
 > line1line2line3line4line5line6
 > ^    ^         ^         ^
 > line7
 > ^

I don't understand: You say it "goes like in 6" but according to your
"^" indicators you skip line5 and line3 in "7.".  Anyway, my Emacs has
for (forward-line -1) the behavior you sketched for cases 5 and 6: line
5 and line3 are skipped when the rear-nonsticky property is not set.

 > The only difference I see is in buffer a when the cursor is at the end
 > of line 7 (actually, anywhere but at the beginning), then typing C-p
 > repeatedly does not skip any lines.  This is regardless of the setting
 > of track-eol.

Please experiment with the attached text file.  You probably need a
visible line before and/or after the lines with the invisible endings.
And it's easier to move the cursor to the end of a line before moving.