Hi hun, just jumping on first of all with some hugs for you, we've been through a lot of Benji's sleep together!

I know how hard you both find routine changes and OT to deal with, so I can't force you to relax about it

but what I will say is that somehow even if you can't relax about it, you do need to accept that OT will happen with this transition. It does with any big routine change but you will come out the other side. If it is any reassurance at all we found this actually the easiest of the nap transitions with J so there is hope.
So back to the beginning.....what was happening that made you decide to cap the nap to an hour rather than the 1h15? What was his routine at that point? Does he have all his teeth now? Not unwell in any way with a cold etc?
I agree with Creations that the 10.5-11h day rule is not for everyone. Days that short did not work for us in the 1-0 and J got through it with some 'rules' that we stuck to regardless of EW, NW, OT. I wonder if something similar may help you? It certainly gave us a sense of being at least a little in control and I think us being consistent really helped J. She was a nap refuser rather than resisting BT or EW but you could do something similar regardless.
Our rules were:
Not to wake in the morning
Not out of bed before 6.30am even if woke early - tried to resettle or at least stay in dark
Offer nap - into bed by 1.30pm
If nap refused after 45 mins or so, get up
If napped, wake by 3 (this would usually be max an hour)
Set BT 7pm if napped, 6.30pm if didn't
If she had one or two NNDs in a row and was clearly losing the plot we would take a quick drive around 3/3.30pm and let her have a 15-20 min catnap if she chose to fall asleep. Any longer she was a bear to wake and would be grumpy until BT.
I think I recall Benji is not usually a car sleeper but has done very occasionally when seriously OT. I would consider giving the car catnap as an option if he is really struggling (bearing in mind it will still be up to him if he sleeps or not) and if he chooses not to, push him to the set BT. We had some 13h+ NNDs when J had an EW but refused to nap but sticking to our plan got us back on track quicker than trying to get her caught up.
I would also say (from our experience, others may disagree) to avoid giving longer naps in an attempt to help him catch up. We found this actually led to further OT due to BT delaying or short nights so it was best to cap the nap short and use EBT if necessary, even if she was shattered.
Hope some of those thoughts are helpful, please keep posting so we can support you x