Wow that's an impressive dashboard that you have!

I can't answer all your questions but hopefully this can help:

1. When you level up to Platinum, does UXP start accruing only from that promotion date onward?
2. Or is there still a hidden pre-accrual mechanism like in the old calendar-year setup?
I've seen weird things on XP counters so I can't answer definitively:
- For someone with Silver status: no UXP appear
- For someone who recently qualified for Platinum: all AFKL flights now appear with UXPs, even the ones that were flown under Silver / Gold statuses. This is probably a glitch as the UXP counter is weirdly stuck at 0 UXP, so take this with a pinch of salt. Also it was on the iOS app, I didn't get to see if it appeared on the FB website.
I'll keep digging but it can be tricky to find someone with just the right timeline: you need someone who has qualified for Platinum status less than 2-3 years ago, as the system won't let you scroll indefinitely in the past.

3. Does a mid-cycle level-up fully reset UXP together with XP, or only XP?
Before you qualify to Ultimate, the UXP cycle is 1 Jan. - 31 Dec. and the UXP counter resets on 1 Jan. as long as you remain Platinum. If you requalify for Platinum, let's say in September, your XP counter goes down by 300 but UXP counter is unaffected until 1 Jan. or until you qualify for Ultimate. NB: I don't see any trace of the reset on 1 Jan. in the history on my account, I just remember that it happened and my current UXP counter matches this.
When you first reach Ultimate status, if you qualify before the month of December, your qualification period becomes 12 consecutive months, similar to lower levels. When you qualify for Ultimate for the first time, your XP counter is unaffected: 0 XP deducted, 0 XP added in the surplus. Your UXP counter becomes your primary counter displayed on the app / website and it behaves exactly like the XP counter. At the end of requalification period 900 UXP are deducted and you receive the surplus.

4. How does UXP rollover behave when your cycle gets shortened because of a promotion?
It behaves exactly like the XP counter. You have 12 months to requalify, and on the date of requalification, you get the classic - 900 UXP + N UXP surplus

5. Any odd cases where UXP behaves differently from XP during promotions or requalification?
I have seen one odd case described above with old AFKL flights that have UXPs. I don't know if it's an isolated instance or if someone else has seen this?

Let me know if you have other questions!