You are putting way too much faith in the integrity of the exchanges and third part wallet services.

There is nothing that stops Bitstamp (deliberately or via subversion) from giving sellers infinite paper btc to trade with. They can get away with it as long as most people don't try to withdraw their coins. If they suddenly started making it hard for people to get their bitcoins back, that would be a possible indicator of this.

There is also nothing that stops Coinbase from loaning out customer btc deposits to short sellers. We'd never know if they had done so.