I totally KWYM - we have EXACTLY the same issue, though our WU's aren't generally quite as early as yours. O rarely resists BT either, even if the A is too short, but we will get EW for that, & I think this is b/c he is genuinely tired from the long day (so goes to sleep easily), but not tired *enough* from the A time from nap-BT (so sleeps a shorter night) IYSWIM?
WRT the routine not sticking for long, we have also had this problem. But TBH I probably am not pushing him to stick to something solidly for long enough b/c as soon as I see he's getting OT I either do EBT or an earlier nap to help him catch up, and he does, but then we are back to square one with early wake, early nap, early BT.
So for the last week I have held him out to naptime regardless of how tired he is & believe me, he is VERY tired. And he has been doing shorter naps too. I have still played BT by ear, aiming for around 5hrs A, but some nights he's struggled to settle through OT, and other nights he's chattered away. Last night I did pull BT earlier & it paid off with an 11hr night. If it helps, our day today went:
WU: 5.35, nap 11.55 - 2pm, BT: 7.00.
He has been incredibly tired this avo & was rubbing his eyes over & over & kept laying down on the floor, but has still kicked up a bit of a fuss once in bed so even with the long day I am still worried he wasn't quite tired enough to give us longer than 10.5hrs tonight. I am persevering though for a while longer & will gradually start to shift BT out so we hopefully get a later WU.
I am thinking this could be a good plan for you: Keep his nap at 12.30, cap at 2hrs & start gradually shifting BT later by 15mins every week until its solidly at 7pm. Be consistent with your naptime & BT regardless of how he naps (pulling it earlier by 15mins max if absolutely necessary) & power on through any OT you encounter. I seem to remember you had success with a 12-2ish nap & 7pm BT before when you first went to 1 nap didn't you? It might be he needs pushing even further to a 7.30pm BT or a 1pm nap but those things can be done as the next step in the plan. What do you think?