Hello, welcome to BW forums
Great you had such quick success with shush/pat - what a shame to hear things are tricky for you again.
A couple of things which come to mind as possibilities for the "regression" in sleep training:
1. often on day 5 of ST people see a regression, LOs last ditch attempt to see if the rules are always going to be like this or if they can get it back to how it was with the props which were previously in place. Hold firm, support him of course but keep on putting him down in the cot for sleep (I'll add below a tip which may help)
2. With a longer morning nap it is possible that he needs a longer A time to help him get a decent afternoon nap. His first A time was 3hrs and he was sleeping a short nap, his first A time is now 4hrs and he is sleeping a longer nap of 1.5 or 2hrs. It could be that he likes the longer time. Increasing the second A time could help with the nap resistance. The crying could be him trying to tell you that he is not yet ready for sleep. You could try an additional 15 mins A time (3hrs 15) and see if this helps over a few days.
3. Some times LOs realise that they sleep better without you (because they are confident of your support and they know you will always return when they need you) so crying can be an indication they want you do go away and let them get on with it. It is confusing and I am not suggesting you leave your LO to cry alone, we don't agree with that, but it might be worth an experiment to put him in his cot and say "have a nice sleep, call if you need me" and just walk out. If he calls or cries then of course you can go directly back in. I did this with my DS because my presence really bothered him. There were also times when my DS needed me (illness, teething or whatever reason) and wanted a pick up and cuddle but would also scream to be put back down because he knew he was really tired and he wanted to be in his bed sleeping.
We pick him up when he cries, calm him down and set him back down as many times as may be needed. But what happens is that he ends up falling asleep in arms when we pick him up to calm him down. So it's almost the same as before.
What you could try is after a couple of pick ups instead of fully picking up you position your hands as though you are picking up but only raise him a couple of cm from the mattress, he feels his weight lifted, feels the comfort of your hands firmly around him but you are close enough to the mattress that when he nods off you can get him down gently but really quickly. If he gets to the point of nodding off the second you pick up even a cm off the mattress then barely lift him at all, firm hands on, raise his weight only slightly and put back down but keep the firm hands on him. Wakes again - then firm hands on him but don't lift. This way you should, if you are quick and alert to what he's doing, be able to get him on that mattress before he nods off, even if your hands are still on him to begin with. Next step is to stay alert to what he's doing again but hands off before he nods off.
It's as if he now completely rejects his crib.
This can sometimes be cause by OT but looking at your times my hunch is that it could be a little bit of UT (under tired). If he screaming blue murder and rejecting the crib you could try taking him out of the room for 10 min then going back in. Just say "looks like you are not ready yet, okay lets go and look out the window for a while".
Your current A times are within the guidance times but these are only guidance and some LOs do better with a bit less or a bit more than the guidance.
eliminating the second afternoon nap. Maybe he wasnt ready for this.
At about 6 months that last CN drops to leave two long naps so it is not unheard of for a 5.5 month old to move to 2 naps. The only thing at the moment I'd say is that the second nap is not really long enough and I would expect him to need two proper naps of 1.5-2hrs and that with two good naps the nights should also get easier. Taking into account though that he was getting through the day on 3 naps of only 45 mins he is still getting more sleep than pre-sleep training.
6am - wake and feed (he wakes earlier than before, we assume because in general he's better rested now)
Yes it could be. There are a couple of other possible reasons:
1. his BT is earlier which can lead to earlier WU - he was doing 11hr nights and is now doing 10.5hr which is not hugely different. 10.5hr of better sleep is better than 11hr of lots of NWs
2. His A time before BT might be too long, it's around 3hr 30 min so although BT is earlier he is getting more tired before BT and could be a bit OT. OT before night sleep can cause multiple NWs and also earlier waking (as in waking before they have had enough sleep). You could try to reduce the A time before BT by making nap 2 a bit later but also longer.
3. Possible teething pain - teeth can move quite a bit before you see them cut the disturbed nights and earlier morning could be due to pain but because you've recently sleep trained it causes a confusion over the cause of disturbed sleep. If there is pain LO can wake up earlier than usual because they come into the lighter sleep, feel the pain, and then can't get back to sleep. If there is a chance of teething you could try meds at the 4am NF (night feed) to see if it helps bring WU later.
No clear cut answer I'm afraid as every baby and every routine differs, but hopefully a few ideas there of how you can go forward. Keep in mind just how wonderful those few days were when things all seemed to click and go well - you can have more of that ahead if you keep going with the gentle and respectful sleep training methods.