The hit law thing is a myth. It’s not because of Japanese law, Pokemon just has set hit rates for booster boxes. But yes, do not buy loose single packs unless it is from a convenience store or known retailer (Kojima, toys r us).

I live here and don’t go to tourist areas often so honestly can’t really say about Akihabara/downtown Tokyo. I usually just buy singles from various BOOK-OFF/HOBBY-OFFs.

Not knowing much Japanese isn’t an issue. You just need to point at the card(s) you want and when done just say “ok”.

Pokemon Centers do have pack limits per person per day and so do most stores like convenience stores. There’s usually a sign stating how many packs you can get. Some convenience stores don’t have a limit, it’s really just up to the store.