It is possible that he is LSN, especially as it sounds like he is happy and coping with short naps. If he is, then I think you might be looking for him falling asleep easily as a sign of having found his current A time, rather than length of nap.
Wow- he's napping in the room your 3yo is playing in too? I am impressed at his ability to drop off to sleep like that. MyDD is way too nosey to fall asleep when her 3yo brother is around and my DS was/still is "touchy" so needed a good long wind down and total darkness to sleep. That said p, I have a friend who could put her little girl down on a mat in the middle of a baby yoga class and she would go to sleep. Thinking about it though, the nap refusal yesterday might be because he is tired but wants to stay and keep watching his big brother! And he might not resettle because he wakes and wants to join in fun again!
It really is all trial and error- you could try taking him upstairs for a nap and following the 4s routine and see if it makes a difference. I do think as babies get older they do get to a point that they need to wind down and be in a sleep environment but only you know your baby and whether he might be at that point now or not.
For timing when to start the WD, that all depends on your baby and is trial and error again (sorry). My touchy DS needed to start WD before he yawned, if we waited for tired signs, it was too late.
You said yourDS is more like a textbook baby though. My DD is too and if I try a WD before she yawns or eye rubs, she'll be UT...so if he's a textbook baby, maybe try taking him upstairs for a short WD when he shows his tired signs?
I'll see if I can find another pair of eyes re feeds
Xxx