That's great you've got two lovely naps there
When you drop the 4th he doesn't have to go straight to 2 full naps plus a catnap, he could take a longer 3rd nap if you feel he needs it. I wouldn't be surprised if he gets quite OT with those long A times at the end of the day which could catch up with him leading to early NWs etc, how do you think he's handling it at the moment? How are the nights? I suspect the 45min 3rd nap is OT, maybe putting him down earlier for that could result in a longer 3rd nap and then he'd be more able to handle the last A time before bed. But I don't want to make you change anything if it's working well
You'll want to push that 2nd A further eventually, keep trying, I know it's hard when they're showing sleepy signs but part of that could be his body clock telling him it's time to sleep, but if you 're able to stretch him without him getting too wound up then as you say, he might start doing a longer 2nd nap.
Well done on pushing that morning later, I really think you'll soon be out of this awkward phase, it's lovely when baby goes down for a morning nap with toddler out at playschool/preschool
That's slightly easier if DS1 will be with your mom, great that she can help out, that should make a huge difference and make it feel more like a holiday
I'd certainly try the white noise, we didn't use it when DS was in our room and I slept terribly, whether or not he did! But DD is still in with us as we're not brave enough to move her into DS's room yet, and it's great having the noise on, we worry less about waking her up turning over in bed and probably get less disturbed by her little noises too, whilst definitely hearing the louder ones