I gotta ask: have you ever
actually used per session mechanics? This all sounds highly speculative and (playing a game that makes frequent use of per-session abilities) quite at odds with reality.
IME, they work better than per day and MUCH BETTER than per encounter.
I've never once had to "guesstimate" the "average amount of kibitzing" and I don't have to worry about "camp time cycles." The game flows much more smoothly for it IME.
Per-session tracking:
- eliminates inter-session tracking
- encourages variety, by making the character to resort to different resources across the session.
- eliminates the need for precise time keeping and lets you focus on game play
- de-emphasizes the continual camp-and-rest thing.
- helps naturally build tension towards the end of the game session.
As for problems of session length: a game presumably knows how long a session is and shares that assumption. If you vary significantly from that it's prudent (and not at all difficult) to adjust the per-session allocations accordingly. Your suppositions about the problems with per session are in reality trivial to deal with, and well worth their advantages.