1. If you are being excluded based on something you can not easily change, you must find new friends. People who care about you will not dis-include you. I get invited out by family and friends who know i am not a "normal eater". Sometimes ill just get a drink and bread. Its about the company, not the food.
2. I would not worry about that. They do not have to have the same diet as you, and MANY people have some type of ARFID even if on a small level. Do not limit your choices to only people with ARFID, let in people who do not have ARFID but will be supportive of you.
3. and 4. You may need supplements (vitamins). You should do research and maybe talk to a dietician or someone similar to get you on a healthier plan. Someone who understands ARFID (very important).

Take care.