To answer your questions from my experiences:
Spirituality issue: The original founders of AA used the Christian base. Some people use the group as their HP. I heard a speaker talking about someone who was so frustrated because he was an atheist. In working with him she substituted "good" for "god" in the literature and it worked. This is not a one size fits all program. But - there are humans involved, with their failings, so there can certainly be people who think they have the sole interpretation and everyone else is "wrong". I like the sentence in the Big Book that includes: "deep down in every man, woman, and child is the fundamental idea of God." To me that means that like goodness it is within us not floating in outer space
Participation: Most people are sensitive to the fact that a person was most likely isolating in their disease and needs time to be comfortable. Simply say, if it something like going around the room and sharing, "I will pass today and listen, thank you". Often people will give their phone numbers or email so you can get your footing by speaking with them individually rather than to the whole group.
Food plan: This can get a little odd in my experience because food is not like alcohol - you still have to nourish your body. Some meetings can feel like a "diet club" and that is not the point. Abstinence is broadly defined as abstaining from the eating behavior that takes you into the category of compulsive. In the beginning, for me, that was most carbs and I had to cut them out. Once the physical cravings were gone I was able to transition into a more broad refraining from overeating. Kind of a vague answer? Listen to the members that speak to your heart and consider them as potential sponsors.