Ideally a nap of 1.5 hours at least as this is likely to be restorative. Having said that, some LOs just don't nap so well and if you're getting shorter naps but nights are undisturbed I wouldn't worry too much about the length.
If you get a really good nap - say 2 hrs plus, you can go for a one nap day and just go for early BT. If you get a shorter nap you may need to watch tired signs etc for a good CN time.
As you're trying to break the need for the chairs at nap time, I'd avoid using them for CNs if you can manage it. But for now focus on one nap in the cot and perhaps use another APOP method for the CN if you need one (we often drove around for 15 mins or went for a buggy walk if E really needed a CN but wouldn't take one.
Re: the time for CN, it's a hard question to answer really. My DD always did really well with a CN and then a super short A to bed (2 hrs) at this stage of transition. Your boys may not manage so well. But remember, a short CN doesn't restore energy the same way a longer nap would so a 1hr20 nap finishing at gone 4 might be worse than a CN of less than 30 mins finishing at nearer 4.30/5. But you know our children best. If you think this will disturb night sleep it might be best to try for a CN earlier in the day (maybe 3/3.30) and trying for a period of time before getting them up & trying again in a bit if they don't sleep. You will be able to get rid of the second nap soon I'm sure but not all LOs will cope well with dropping it immediately. Z may cope better as he is textbook type and they manage slightly better with OT, but S may find this tricky, so may well be worth you working to keep a cN for a little bit, even if not everyday.
Re: winddown. I would keep this reasonably similar for BT and naps, & definitely keep your sleepy phrase exactly the same for all sleeps and wakings when you're trying to resettle.
Your ideas for winddown sound good to me - it's a little difficult to say though as different babies & children need different things. My DD for example likes a super short winddown - we tell her it's time for bed and then go upstairs saying goodnight to a few objects on our way, then close her curtains and say night to the garden then lay her in her sleeping bag, and say we love her and will see her in a bit/in the morning - sleep well. This works for her but I read some other mums doing half hour winddown each time. I'd stick with what you've been doing as it seems to be working so far
I'm glad your mum has offered to help, that'll be a great support I'm sure. Here to hold your hand as you do this xx