If I'm going from USAR to AD, do I have to adhere to the current business rules or will I be able to pick from any under-strength MOS?

I just went through this process. The recruiter can't help you with a job, that will be solely done with the meps counselor. Once your grade determination comes back, your recruiter can tell you for sure what's open in accordance with the business rules, it's the same options you will get at meps. As far as I know, you are going by June 2017 business rules, I haven't seen any 2018 ones?

It depends on how hard your recruiters are willing to work with you and how on top of it you are. When I went back active I was told I would have to reclassify because my MOS was balanced. But I checked back weekly if not monthly for updates on openings. It took maybe a 2 or 3 months longer than expected but I kept my MOS and rank only draw back was I got stationed at FT. Hood. So that’s the only caveat is that if it isn’t an under strength MOS you have to take needs of the army.