I'm not sure I have answers on everything, but here are my thoughts on it:
1) Gro snugs. We have one of these, but if he likes it and we use them full time, how many would I need? They only go up to 12lb, so don't imagine they last long, so:
I've not seen these before, and may get a few for use with DC2 who is due in about 3 weeks. I would imagine you'd need at least 3 - one on, one washing, one drying.
2) Would a swaddle blanket be better? Either the gro-swaddle, the big ones from Aden and Anais, or similar? But then, isn't it a pain in the arse to have to unswaddle and re-swaddle for every night-time nappy change?
Not sure, I think if you get used to swaddling then you get quicker at it, much like nappy changing! The swaddle may also save a change of sheets should there be a nappy leakage incident, so that might be an advantage.
3) And do the above things get covered in poo? (Planning to use cloth nappies, so leaks are a concern). If so, would I be better buying swaddling blankets rather than the more expensive gro snugs, for laundry rotation?
Really depends on your baby, the nappies and so on. If you're getting lots of leaks it probably means the nappies aren't the right fit.
4) Is the alternative to these just a cellular blanket up to nipple height? Do they need a sheet too? (autumn baby)
Yes, you would use cellular blanket(s) instead of a swaddle. I just used cellular blankets layered as necessary, then when colder, more layers on the baby. When your baby is big enough gro-bags are great.
5) I have some second hand Gro bags, but understand they can't be used for newborns? Yes, there is a minimum weight for them I think.
6) Do they wear sleepsuits (with sleeves and feet) underneath the swaddle, or is that too hot? Are sleepsuits only for if they're sleeping under a cellular blanket / sheet?
It depends on the temperature of the bedroom. The Gro website has advice about this here:
gro.co.uk/faqs/about-the-gro-swaddle
So depending on the temperature, they might wear a bodysuit (short sleeves/no legs) underneath, or a sleepsuit (long arms and legs) or both together if it is colder.