Char I think those 12h nights are catch ups, it's normal for them to seem much more tired once the catching up process begins as the edge of the adrenaline starts to wear off. There's no way on earth he was UT if he fell asleep like that, I would have either just let him sleep at that early time or gone for nap at normal time and basically ignored that 10 minute mini doze. I don't think you can just say at this age 'oh that was a 12h night, he must be well-rested' because it's not just the night/nap before that counts, it's what's happened over a few days or even weeks. You also woke him this am which says to me he still needed more sleep to be fully rested even though it was a 12h night.
Remind me (sorry I've lost it somewhere in the thread) what the plan is with set nap and BT? Are you doing nap 12.30 and BT 7.30pm? I just wonder if that's pushing him too hard on a background of short nights and poor napping...for comparison we do WU 6.30/7, sometimes a touch later but wake her by 7.30am, nap 12.30ish up to 2h but more often 1h15-30, BT 7pm. How about pulling in bedtime earlier for a while, stick with set nap time by all means to help him set to expect nap at that hour, but keep BT max 4-4.5h post nap?