If I had 1 million MINT, would it be better to have it all in one wallet or divide it into several wallets and mine from each?
It is better to have as much as possible MINT in one wallet. This greatly increases the chances of successful mining and increases the maximum reward for mining.
Has anybody evaluated the online wallet to see if it is safe or it can leak the private?
It is safe.
Has anybody evaluated the contract code to see if it actually does what is claimed?
Check it at Etherscan.