Ginger, It sounds like things are pretty stressful and that you are overwhelmed. If you can afford not to take on the substitute teaching, you might want to put it off. The unpredictable nature of sub-ing seems like it would add to your stress. Then again, having extra money can ease other types of stress. That's a tough one.
I also have 3 kids, one of whom is a gifted dyslexic (she also is dysgraphic). At the beginning of last school year, I got overwhelmed completely by all of the things that the "experts" told me I had to be doing "right away." School nodded in all of the appropriate places and then gave me suggestions of all the extra things that I could be doing to help her at home (writing practice, keyboarding, guided reading, etc.) I often felt like a total failure because I couldn't do enough for her. I also felt like I was letting my other kids down because I was always spending so much energy on her.
Eventually I realized that I only could focus on one thing at a time with her and that we could only work together peacefully for 30-45 minutes per days -- some days not even that. I needed to let go of trying to do everything. Things aren't always perfect but my stress level has gone way down. I also think that my DD's has too. Can you pick one manageable thing and let the rest slide for now?
As for the "anti-gifted" people, I wouldn't waste too much time and energy on them. If they can't be supportive, they are just adding to your stress. Are there any other parents with kids in the gifted program who you could talk to? Hang in there.
