Phew - lots of questions but great that you're thinking about all of these things already .
I've had my girlie for 9 years now, have a 20 month DD and work full time so I can give you a little insight into how it's worked for me.
Pre-baby I was riding about 5 times a week, competitions at weekends etc. etc. When I fell pregnant with DD I had already put my mare in-foal so the decision on whether or not to ride was out of my hands. However, me being ready to get back in the saddle happily coincided with the foal being weaned so it worked out rather nicely. The youngster has since been turned away & comes in occasionally to learn some manners, but is otherwise pretty low maintenance.
While I was on mat leave setting a routine really helped me. I keep my mare on DIY-assisted livery so the yard turn out for me and I do everything else. I aimed to get to the stables for 9:30 every morning and again at 4pm which worked brilliantly in training DD to nap at those times. It also stopped me from getting cabin-fever from being cooped-up in the house for days.
I also have a brilliant & understanding yard-owner who let me park my car close to the stable so I could hear DD if she ever woke up. YO would also babysit at least once a week while I went for a ride which had the added benefit of getting DD used to being left with other people.
Now that I'm back at work the yard turn out & bring in so I only need to go up once a day. I always go in the evening, sometimes with DD & sometimes not (DH does bedtime & one day a week does the nursery pick-up which really helps). I get to ride about twice a week but could manage more if I was a bit better disciplined.
I love being at the stables & find I get really ratty if I don't get my quota or riding in - it really is my stress relief. Being around horses has also been great for DD - it's practically her second home & she loves it there. She has her own wheelbarrow & tools, loves watching & patting the horses as well as playing with the cats & dogs and chasing the chickens....she's turned into a real out-doorsey, rough-and-tumble toddler which, in my book, is brilliant. One day I expect she'll want her own pony & I can't wait .
If you can cope financially with keeping the horse then do it - there are ways and means of making it work and it doesn't have to cost the earth. Plus, being outdoors & in the fresh air will be good for both you and your baby.
So, now I'm expecting DC2 (8+3 weeks) and have already decided to turn my girlie away as soon as I feel too big/uncomfortable to ride. She's 17 now so a winter off will do her good - she's a horse after all, standing in a field & eating is what horses do .
Quite a lot of waffle there but hope it helps. Good luck with whatever you decide and keep us posted.