Can a double spend attack be obfuscated by mimicking the transactions seen before the fork?
The short answer is that there is no way to double spend "without arousing suspicion". Presumably, you would be double-spending against someone and that person would notice as soon as you diverted the main chain. 

longer answer:
As long as you're building your private chain you can double spend everything you have by normally broadcasting to the public chain, knowing that it will be reversed later. There is no way to detect/defend against a 51% attack until it happens, and once it happens the security of the blockchain of Xcoin is obviously compromised and the whole coin/your stash of coins is now worthless, so there was not much point in the double spend anyway.