If I can summarise your request:
1a. fixed sized notebox or
1b. grow the notebox for page N after processing page N+1
2. box around one translation.
3. A 'diglot' but not one which is aligned at paragraphs, instead flowing 'normally'.
4. All notes in one, full-width area (from both translations)
(4) is possible, I can't remember where it is in the user interface, but the switch is \diglotSepNotesfalse
The answer for 1-3 is somewhere between we can't do that and we can't do that yet.
(2) is a strong desire, but it demands a rewrite of multiple bits of code. Some of those changes are likely also needed for a new (currently-in-development) polyglot-complexpages plugin I'm slowly chipping away at.
(1b) isn't possible with any of the current code. It might be possible for polyglot-complexpages. Given the amount of re-juggling you're implicitly asking for it will be slow and there will be horribly messy situations with no easy answers: Imagine page X and X+1 are facing each other, but X+1 has a long note on it's last line. We grow page X's notespace so that it matches X+1, but that moves a line of text onto X+1, which in turn forces the long note at the end of X+1 onto the page X+2, so that now page X and X+1 have needless blank space and readers look at it and say "That's horrible, why did they do that?" I'm tempted to say it might be theoretically possible but I'm not going to go there.
IF, HOWEVER you tell me I can build a single block of notes from the combined notes from pages X and X+1, and then split it exactly in half, ignoring where the notes actually appear, then it's much easier, but someone probably gets lynched by the world's typesetters.
(1a) is confusing to me. Are you thinking you can guarantee that there will never be more that N lines of footnotes? How should the code react if you're wrong? TeX Does have something similar that would force a pagebreak within / before the footnote if the note takes too much of the page, but I expect it will only work with a very specific set of settings. (non-paragraphed notes being an absolute must at the moment, and probably no separate cross-references).
Adding additional space to the note area isn't possible yet, but would be a fairly trivial change to make.
(3) Is something that is probably going to be possible with the above-mentioned polyglot-complexpages plugin.
