I think you've done a lot of thinking around this and you know him best, so yes definitely go with your mummy instincts
I have definitely seen long NWs caused by OT so that seems very plausible to me.
Bear in mind that day when he did the lovely long nap was on slightly less than 4h A time - that 1h 20min nap on 4h A could well have been OT, or of course hunger could have contributed.
Another thing to consider, just based on that other day when he slept better at night - you may want to go for a morning CN (UT, eg 3h A, and wake after 30-45min) and longer pm nap on days when he seems very tired in the morning or has had an especially bad night. You'll have to play it by ear a bit, but many LOs find they need to chop and change between one nap and two nap days for a while, often quite a long while, before they're ready to drop the second nap completely. Generally they need to be able to manage 4.5-5h A consistently, I think. Also bear in mind that two nap days often will have less overall sleep than one nap days, I certainly noticed mine slept a little more overall after dropping a nap. It's like they needed more total sleep to make up for the increase in A times from dropping the nap. So he may need 14h45mins on one nap days, but a bit less on two nap days, if that makes sense?