Yes, but the miners can use serveral usernames and password, to avoid the hash blocking.
I wasn't personally mining, so can you please provide a source for proof that it's effective, for both private farms and rented hashpower? It's something so obvious that I think they would never even try to enforce the limit, if something so simple would make the limit meaningless.
Yes, but by this way you are taking the risk of mining in a pool of the same devs of the coin. So the coin won't be plural and fair.
Yes, that's true if the project is a scam. Minerals wasn't. So, if the people behind the project can be trusted and prove their idea has potential - it's not too much of a problem.