Hello, sorry to hear about your Mum and step-Mum, this must be hard. Of your 3 options, personally I would prefer sheltered housing/assisted living for one of my relatives. Swanky retirement villages (around here anyway) can seem to sell a dream of active retirement living but are super expensive to buy into, have really sky high service charges which relate to things like maintaining swimming pools which some of the residents never even use (and if you are a swimmer and able to get about a bit, you can buy gym membership much more cheaply compared to paying £££ for one on site). Plus most seem to only be suitable for people who are mainly independent and don't always provide (or easily provide anyway) additional care and support if the resident has some issues and starts to need personal care and help with things like washing/dressing/medication. Around here anyway they are also all in villages or right on the edge of town which just makes life that much more difficult for shopping, healthcare, socialising etc. compared to being in the town itself (even if step-Mum drives now, it's wise to plan on that not being the case forever). I've known several people struggle to sell on retirement flats in these villages once they are no longer brand new as well.
My (disabled) Mum has recently moved into a really lovely brand-new sheltered housing complex and we are really pleased with the decision so far. It's been really nicely done (doesn't feel institutional or like a care home, which TBF some of the older complexes do, this one would feel more like a nice hotel or a upmarket spa to walk around), it's big enough with something like 200 residents that there is a good social scene (helped along but not forced on anyone by on-site managers), there is a cafe, restaurant, hairdressers and several nicely looked after garden spaces on-site (but crucially nothing like a gym which would add £££ to maintain), residents can also get involved in the gardening if they wish. It's in the town centre which may not be exactly what your step Mum wants if she's used to be in the countryside or a village but suits us all very well for her being able to still visit her friends and family/have visitors (after lockdown!), go for a walk to the park, out shopping etc. Crucially there are on-site carers 24/7, you don't have to buy any extra care if you don't need it (or you can just have cleaning/laundry/shopping done and no personal care), but it's really comforting to know that if things deteriorate and she does need more help, she can hopefully stay there longer term compared to somewhere which would basically shrug its shoulders and say 'our complex is only for people who are independent'...
The live-in option, I guess it could work for someone who is still relatively able to look after themselves, but I would be a bit cautious, I think it's a lot of trust/responsibility to put on one person and you would need to hire very very carefully, and set really good boundaries which can be hard with someone living in. Everyone would need to understand and agree what the carers duties are and when they are 'on duty' and when not. E.g. if they are expected to do a full days work doing all the housework, cooking, shopping and errands, I don't think your SM could then reasonably expect them to take her out or sit with her/make cups of tea etc. in the evenings as well - but if SM is getting forgetful or is a bit demanding/lonely I can well see that might be hard to understand. And what would happen on weekends, or if there's an issue overnight - you can't expect someone to be on-call 24/7 just because they live in? Plus as you say SM is very sociable, just having the one person around all the time might get a bit jarring/claustrophobic, even if you strike lucky and find someone she gets on really well with (not a guarantee, just because someone is a good carer doesn't mean they will automatically 'click' with SM), being with only one other person all the time would test the patience of a saint IMO...
I think if your SM really wants to stay at home, I think what I would actually try is getting a couple of different people/services in to support her, as far as money allows. So maybe in addition to gardener, you could get meals delivered, a cleaner, send laundry out or get cleaner to do it, a red button/pendent service for emergencies, and then maybe a couple of 'helper'/'carer' visits a week to facilitate social time - doesn't have to be same person every time, but maybe someone with a car who can help her get out and about more?
good luck, hope you sort it 