I put my maiden mare in foal and have just weaned the resulting colt. I'll give you my own answers to the questions but everyone's experience will be different!
Echoing above about only breeding your mare if she is well built, and number one for me is TEMPERAMENT. I can really see how my foal, Loki, has taken on mum's personality, which in my case is great as she is super laid back and easy to do anything with. I have even had others comment on her great nature and how he has taken after her. I would NEVER breed a mare who was nasty, hard to handle, nervous, etc.
1. I wouldn't think her age would be a problem. My mare was 7 (I think?) when I put her in foal. But 11/12 isn't old by any means.
2. The only issue we had with my mare was the first time we took her to the stallion (Live cover) she didn't want a bar of him. Wether it was because she was a maiden and didn't know what was going on, or wether the timing wasn't quite right (I'll address that below). However second time we took her it all went well. I did a heap of research and had knowledgable people around to help me out in case there were any problems - But luckily for me it all went pretty much by the book. I don't think it is meant to be that easy for your first foal! But having a good, quiet, well handled mare, in a safe place with experienced people, made a big difference I think.
3. My mare was the same. I have never noticed her go in heat and so I took her to the vet to be scanned. They advised she would be coming on the next week so off we went to the stallion - But as I said above, no dice, she wasn't interested at all. So I took her back to the vet the following week and they gave her a shot of prostaglandin which brought her on again and a few days later back to the stallion where she stayed for a week, being served every second day, total of three or four servings. I also got her checked over for general breeding soundness with that first scan to make sure she was ok to breed and didn't have any issues or infections. I then had her scanned at 14 days to check for twins (None thankfully) and again at 45 days to make sure she was still in foal. That was it then as I went interstate and left her in the care of experienced friends.
4. Tis is up to you, but I would just maintain her routine. I still rode and competed my mare for a few months after getting in foal - We campdrafted and went to clinics. If I had still been here I would have kept light riding up until she let me know she was uncomfortable - Staying fit is hugely beneficial as it keeps the muscles toned and keeps their weight manageable (For porky mares like mine!!). However I wasn't in a position to keep riding her so she just spelled. A few months before her due date she moved from her normal paddock to the place she was going to foal down to ensure she had enough time to settle in and build up antibodies. She was on her own to foal and then put back in with other horses a few weeks later.
5. I wouldn't stable her. My mare was/is the same and I didn't want to add any stress. She was in a clean, grassy paddock next to the house and I camped out there when she was due. She went on the first night I camped out - She had a foal alarm on and I still missed it. Woke up at about 5am to a strange whinny, looked out the window and we had a baby! Just went down and checked them both over, made sure she passed the placenta completely, hung around to make sure he had a drink, then left them alone. Loki was down on his bumpers in the back so they spent a week or two in a yard (Not a stable) while he got stronger, which worked well as it made it easy to handle him. Again totally up to individual preference but as I was going interstate again for a while I had him leading and picking up his feet by two weeks old, and he has been great ever since.
My best advice would be do as much research as you can, know your mare, and try not to interfere too much. I have been lucky and everything has gone great, and Loki is an absolute delight to deal with. My mare is now home and coming back into work and Loki is out learning how to live without mum with some other weanlings as company.