Hi there, if there are no props as pp suggests then the key for independent sleepers is the correct routine & A time. They know how to get to sleep on their own so the hard part is done; it's just finding the A time that will get them tired enough to sleep through 2 cycles. I would try pushing the morning A slightly, say 10 or so minutes like 2 hr 40 and see what happens then. You say he's going down ok and going off to sleep on his own so he's tired, but not quite tired enough.
If he only does a 45 min nap and you can't extend then you need to reduce the following A time, or he will indeed end up OT. Doubtful he would be able to continue at 2 hr 30 or more on short naps. Then add in the NW and yes a big OT cycle. You may need to clean up the OT before extending his A time. Teething also usually causes a reduction in A time so sometimes you just need to step back from EASY and follow cues until they get over the worst of it.
Re: pain meds - I agree; I hate giving them too but at times they have really helped and have done no harm. It's certainly a personal choice but maybe worth a discussion with your paed if you think he is genuinely in some discomfort from the teeth.
About the CN, some days we are doing the CN quite late (he only does 30 mins) and then we push bedtime out a little if need be. Like we CN him at 5:30 - 6 and then we will follow his cues to bedtime, usually he's only good for an hour or so after the CN anyway.
We had the same problems between months 3 - 6, basically short naps started out as developmental but then progressed to being an issue with the A time. He had a huge jump in A time between 5 - 6 months and I didn't catch on until he was in an OT cycle. Got help on the EASY board and now we are getting better naps when the A time is spot on. It's just really a spot on thing for a lot of LO's - 5 mins one way or the other can spell disaster.