Huge hugs to you!!!!!!!
At the risk of contradicting everyone, and when you want to try something else, I would urge you to try reducing his A times throughout the day.
The reason I say this is
a) your LO is very overtired and OT takes awhile to get out of. It won't be fixed after one long marathon nap.
b) it seems to me after I've skimmed this post, that you've had the most success after short A times
c) short naps are part developmental at this age, but they're also related to OT. Extending A times with an OT baby is going to cause more and more problems. I speak from experience.
d) your DS is still very young and some LOs cannot handle long A times
My suggestion is (and it's only a suggestion-take it or leave it) is to only keep him up for about an hour and a half through out the day. Maybe even a touch less-I know, I know-that is really short!
I'd reduce his A times for 4-5 days and see what happens. If it doesn't work, then at least you tried. If it does work, then you could try slowly extending his day/A times out again.
I'd also suggest not going in right away when he wakes at a short nap. Wait outside his room and give it a minute or two so you can assess what you hear. Once he's caught up with the OT, he may start to transition by himself and you in there might be interfering with him learning how to do it himself. He may also only mantra cry instead of hard crying so it might be easier to let him be.
If he takes a decent nap-anything over an hour, then I would consider it a good nap. I never consistently got the ideal '1.5hrs' nap and it drove me crazy. I was anxious and was stressed all the time. I tried extending naps all the time and I tried extending A times and it backfired with EWs and NWs.
Hope some of this helps!
Hang in there!!