Hi Serenity, Welcome to the forum! I'm entering this thread a little late, so what I have to say may have already been mentioned. Please forgive me if I'm repeating anything. I read some of your posts, but mostly I only skimmed the last 4 pages.
First off, if you find people are being rude, don't take it personally. Just report it to the moderators. Just because there is a disclaimer that breeding is a touchy subject, does not give people license to be rude. Also remember that emotions aren't conveyed through written word (unless there are emoticons used!) and so it's difficult to sometimes interpret what people's actual intentions are. A forward or direct reply may only be just that, with no malice intended.
Now that I've gotten that out of the way, let's talk about your mare. Even if she was only with the stallion for a few hours, she could very well be pregnant. An example I can give is my mare, who I intentionally bred this year. She is a 12 year old maiden, and was only covered twice by the stallion. When I took her to the vet for an US, the vet was 99% positive, that based on the size of the embryo, she caught on the very first breeding.
If you're set on keeping the baby, then you're taking a step in the right direction to learn what you can. If you're planning on getting your mare ultrasounded to know for sure, also find out from your vet what the potential costs are. Things such as mileage, after hours dispatch in the case of an emergency, basic examinations, etc. will at least give you a baseline as to what you could be incurring for vet costs. Don't forget vaccinations, deworming, etc. Your vet should be able to provide you with a schedule of when you should deworm, vaccinate, etc. Your vet should also be able to tell you about feeding and vitamins. A lot of feeding is based on mineral levels and plants in your area. Some areas are extremely high for things like iodine or selenium, while other areas are lacking in those minerals and so you would need to supplement. The type of hay available to you will also determine if and what you need to supplement.
The other thing you should do is research, research, research! Buy books, subscribe to magazines, talk to knowledgeable people both online and in person. I find a handy resource is thehorse.com. It's a free subscription and there is often some good information in there. I also have a subscription to Equus magazine.
As someone mentioned earlier, find out as much as you can about your mare's history. I know my mare's entire history and I still had her genetically tested for the 5 common genetic disorders found in QH's (HYPP, HERDA, etc.) as well as Lethal White. It was more for my own peace of mind than anything and the cost of these tests is minimal when you look at what a potential vet bill might be without knowing.
Lastly, pictures are a must.
Grade or not, she sounds like she's pretty special to you and worth sharing.