Welcome! It sounds like you have had your hands full! I doubt this had anything to do with ear infections, though. I would say there is a lot more going on than ADHD.
Regarding the firesetting- my son almost set himself on fire 2 years ago- he says it wasn't purposeful but it didn't look that way to me. Then, he set a brush fire last year by dropping lit matches around himself while standing in a wooded area full of dried leaves. He was charged and is still in legal trouble (he's done a couple of other illegal things too, but they haven't been as dangerous as that) Anyway, I make sure that he has no access to lighters or matches now. Yes, he can still get them from someone at school or in the neighborhood at times, but he won't get them at home- even if I have to sleep with my only one (I smoke cigarretes outside of the house). I would recommend that you do this, too.
Not only will it decrease his opportunity to set fires, but if he ends up setting one that results in involvement by fire dept and police, chances are they are going to make issue if he got the matches or lighter at home. I was asked about it several times- my son did not get the matches at home, thank goodness, but the way all those people were acting, you would have thought we were talking about a gun in the house instead of matches or a lighter.
So, I would do a thorough check and make sure you get them all out of his access- tell any and all family members to please keep theirs away from him, too. You could just tell them that he's become very curious about fire setting and given his age, you want to make sure he doesn't have the opportunity. I would not try the "light matches until you hate them" approach with this boy. Something tells me it might not work with him- it wouldn't have worked with mine, I'm sure.
As far as what to tell the rest of the family about diagnosis and medications, etc. It sounds like they might be the type to disagree no matter what. So, try not to buy into thinking that if you tell them more information maybe they will understand. As others have suggested, I think you just need to keep making decisions as you see fit and try not to worry about them not "getting it". Many of us here have had similar problems and hurt feelings over extended family and their opinions. I can only offer {{HUGS}} for that!!