Here is what we ended up going with for XP for Felltower with PCs who are stuck behind gates at the end of a session:
Per Session, Not Per Delve. - Normally, XP is per delve. But if PCs are trapped in a gate-accessed play area for multiple sessions, that's not fair. It's not less fun, but it does slow advancement and I don't like doing that to the players. So if a gate-accessed area takes 2-3 sessions to clear, so be it - the PCs earn 2-3 sessions worth of XP.
Per Session, not Amortized. - Loot and exploration is for a specific, single session, not divided up over three delves.
I count the loot garnered each session and, once it's sold and diveded, pass out XP to the group. When it was looted matters.
In other words, if a group goes through Gate Z to World X, and earns 1000 sp, 40,000 sp, and 0 sp, the loot for a 450+ character is 2 xp, 4 xp, and 0 xp, not 41,000/3 = 13,666 = 4, 4, and 4.
This is critical to avoid spending extra sessions "just making sure" when you know doing 1, 2, even 3+ extra delves won't reduce you below your loot threshold. That's essentially milking treasure found in one session to make up for less in other sessions and earn more XP.
XP comes at the end of the delve. Like it says, you get your XP after you return to town, not after the session ends.
MVP is per session. - Enough said, there. Every session has an MVP. That comes right away.
All other "per session" stuff is "per session." - Serendipity, Wild Talent, etc. - it's per session.
I hope this clears up how we're doing XP for delves such as this last one.