Can ransomware infect iCloud Drive?
In short, yes, it is quite possible for ransomware to encrypt the contents of your iCloud Drive. It is possible for the ransomware to encrypt the data of your iCloud Drive, and then your computer would sync the changes to Apple's cloud, requiring decryption to access the data again.

To answer your questions:


Probably. The iCloud Drive folder is stored locally on your computer in /YourUserName/Library/Mobile Documents. It would be getting backed up unless you have specifically told Time Machine to not back up this directory.
No. Apple does not make backups of a user's iCloud Drive accessible. It is unclear if Apple even keeps copies of user's iCloud Drive, but they are most certainly not accessible to the users.
Yes. If ransomware encrypts the files in the directory /YourUserName/Library/Mobile Documents then the changes would be applied to your iCloud Drive, rendering the information useless without decrypting the data.