Does that 10th erasing attempt also remove the data from the cloud?
10th failed guessing attempt. It does not erase the data, just the key. If Apple is smart (and mostly they are), they will use the same key to encrypt the data in the cloud.
No, it doesn't. However, unless the terrorist paid for an upgrade, Apple only provides 5GB of free iCloud space. Assuming the iPhone 5C was a minimal iPhone, not counting iOS, there could still be about 9GBs of data unaccounted for still on the iPhone.
Apple has already been handed a search warrant for the data in the terrorist's iCloud account and, according to Tim Cook, complied with it and provided copies of everything that was in it to the authorities.
I would expect the terrorist's iCloud data was encrypted with the same key as the iPhone. That's the way iCloud works. It is also possible that he or she opted out of iCloud backup as soon as they started planning the attack and nothing was saved there after that date.
Cook has also stated that Apple has been working with the FBI and other authorities to unlock the iPhone all along. They just have not been willing to create a tool to unlock the iPhone that would compromise their ecosystem. That's why the FBI went to court seeking an order compelling them to do so.