Disclaimer: I'm a dental student.
I think you have a newer dentist who wants to be VERY sure he doesn't miss anything. It's a big responsibility!
At my school, they stress risk assessment, and going with the least invasive treatment possible for each patient. For a patient with no caries in the last year, good hygiene, and regular checkups, they tell us to consider asking them to come back in 9 months instead of the standard 6.
Again, huge disclaimer: I'm a student and therefore know just enough to get myself into trouble! But going by what I have been told in lecture, the frequency of x rays you describe sounds reasonable. Keep in mind that caries between the teeth are sometimes impossible to see without an x ray- and that if they are caught before they become "cavities", the teeth can often be remineralized.
The sealants are pretty common. If they have adult molars, remember those teeth will hopefully be in there for many decades, including maybe some years when they are teens and too busy to brush. On the other hand, if they are coming in for cleanings, the doc will likely catch discolorations before they become decay, and it would not be unreasonable to want to wait. Your kids are not generic "kids who lack motor control", they are your kids, and the evidence of how well they clean is right there! Some teeth have particularly deep crevices, and it can be a good idea to just seal those and save a lot of heartache.
I don't know about the orthodontics... is the extra tooth out of line with the others, or causing crowding? I think this would be something for an orthodontist to look at if you think he might have a point. Both crowding and spaces left for a long time can cause problems that are cheaper and less painful to head off.
It can be hard to tell if a dentist is on the same page with you re: doing things for your health vs. wanting to look like a movie star and/or remove every possibility of risk from your mouth. It doesn't hurt to be up front about your goals, and if the doctor doesn't seem to get what you're talking about, find one that does. It's not unusual, so I hear, for a lot of patients to jump ship when a new dentist buys out a practice.
Mama to: Aster , August, Emmett:, Ruthie: Step mom to Malakie, Cameron , and Aurelia