Isn't a 70k population still too tiny to support the MoR-verse's level of social complexity?

I don't think so. I think 70K population is more than sufficient for what we observe. After all, the primary servant-class (House-Elves) isn't included in that number. The primary banking/financial class and atleast some portion of the manufacturing class is goblins, which is also not included in that number. Anything that concerns the gathering of raw non-magical material (Farming/mining/etc) can be gotten by trading with (or thieving from) the muggles.

The rest: nobility, ministry employees, aurors, shopkeepers, athletes -- I think they can fit in easily in a population of some dozens thousands. Possibly even less.