I'm sorry, I have to revise my previous answer - I just double checked our source code and I was wrong on 2) and 3). The correct answer must be:
2) and 3) Enabling or disabling filename encryption follows the long path restriction for the new filenames. If support for long paths is enabled on the drive, it could produce files which are only accessible with long path support enabled (obvious). If support for long paths is disabled (default), the "warm-up tests" for the operation will fail if it would produce any file with a too long path and the operation will be canceled before it started.
Regarding the other questions:
4) If the plaintext filename are then short enough, the files should be synced by the cloud storage provider. If you then enable it again, I would expect the cloud storage provider to remove the file with plaintext filename from its records and _not_ sync the file with encrypted filename. We're performing a simple Rename operation (which might also look like a Move operation to the cloud storage provider). But this is something out of our control and I cannot make a general statement.
5) Support for long paths has nothing to do with EncFS itself. As long as the underlying file system supports long file paths (which almost all modern file systems do), EncFS is able to handle those files (e.g. on Mac OS X).