Yes and i been syncing vita folders for over 2 years now and i never been banned
for games you already got synced on your account they wont be on your account again unless the timestamps of the new folders are older then the ones that are synced
You can sync them to your PSN account or just any account they dont have any id so you wont need to resign them
just put them in your trophy folder using filezilla and vitashell and then the only thing you need to do is open your trophy app and wait for 10-15mins till everything is synced