Both dropbox and drive support modification times. To set a modification time on dropbox you have to upload the file again, alas.
Once everything is in sync it shouldn't require any more uploading though.
Indeed. It shouldn't be necessary though...
That is correct. Drive supports MD5 - dropbox have their own hash.
You can use rclone check --download - that isn't a global flag so it doesn't exist in the /flags docs.
A debug log (with -vv) would be useful to see why it is getting stuck.