1a) Regarding exchange itself, it serves as your wallet and you own private key so it's much better than other exchanges. Make sure that you follow best practices so that you stay safe (use only on safe pc, strong password, backup wallet safely).
1b) Regarding assets, Open.BTC is an IOU issue by OpenLedger company. There hasn't been any major issue with this company in the past but make your own research, risk is similar to holding BTC on other exchanges. bitUSD is completely decentralized and trustless. In theory it's very very safe but still there might be a bug in software or a systemic failure (like total collapse of EU/USA economy) might cause that it starts behaving unpredictably.
2) I would guess that it will be dramatically lower when selling BTC for bitUSD, might be similar when buying Open.BTC back (it's 0.2% for OPEN.BTC, not sure about Coinbase).
3) If you don't want to completely cash out of crypto world, BitShares is an awesome platform.