Ooh it is tricky isn't it... I was interested to see Saturday's routine actually, it reminds me of the sort of thing which used to work well for my DS. That also fits with what you said about him struggling with long morning A times but easily doing long A times in the afternoon - that's just how my DS was too. So anyway, here's a suggestion to consider - how about going for a slightly UT morning nap, expecting it to be short (or even actively capping it) in the hope of getting a nice long afternoon nap? I found the long pm nap was key for DS as he could then manage a long A to BT, and the combination of those things helped him get a good night. Actually looking at Saturday, perhaps he needs more like 13.5h total sleep? I agree that 12.5h is low, some may do that but those EWs with getting back to sleep afterwards sound like possible OT to me, he may not quite be ready for one nap just yet, or alternatively you may find he can cope well one day but not the next - many LOs go through a phase like that.
This short am nap, long pm nap can be tricky to get the timings right as each LO may be a bit different, but how about something like:
WU 7am
Nap 10-10:30 (wake him if he doesn't wake himself)
Nap 1:30-3:30 (this is where you'll need to play around with A times etc)
BT 8pm
Just suggesting that as it's 13.5h overall, has a long pm A time and 11h night, ie as he did Saturday, but it'll be a lot of trial and error to see what works best. Something to consider, perhaps?