a: I can't answer the reason for 30 votes, it seems to predate Steem back to BitShares. It would seem that a number less than consensus witness slots would be a better option, given what happened on Steem.

b: The number of witnesses was explained by Dan Larimer a long time ago as a trade-off between blockchain security and decentralization and being able to properly compensate those witnesses. The more witnesses there are, the smaller the slices of the witness rewards are.