I don't think there is any right time to go back to work, it's hard. But when it comes to daycare, I don't think it would matter if they're 6 months or 18 months, the first few months there is a great chance they (and you) will get sick. With our first she was maybe 18 months I think when she started and the first 2-3 months we had colds, gastro, she had chicken pox...argh, it was hell. Then my second she just had cold after cold, then croup, bronchitis, felt like everything she could get, she did get.
Going back part time is hard too (well I found it hard, some people it works great for). But I just felt like I couldn't get into my role 100%, I was missing out on things so felt like I was always 1 step behind. Things would be decided in meetings on my days off and I'd hear about it a week later. Frustrating. I also felt like (and I still do) like I can't give 100% at work or at home, that something is always slipping or not getting my full attention. But, over time for the most part I've accepted this and it's getting easier.
Don't stress about the mothers group thing. It sucks that you miss out, but can you ask to do your regular catchups on your day off if you're the only one working maybe? And kids totally develop at different speeds. My first did everything sooo early (I was convinced she was a genius ) and my second was always the last to do everything... but now she's a toddler she's exactly the same as everyone else.
It's so hard going back to work. And the first few months are definitely the worst. If you can try and stick those out, then I think you'll be in a better place to evaluate what you want to do long term.
It probably doesn't help, but I cried every day (on the train no less) when I returned to work the second time for about the first month. It did get easier xx sending you hugs, such a hard time and I hope you know what you're feeling is normal and you're doing all the right things. You're a great Mum, I can tell that by reading all your posts on BH