Hello Suiseki, Thank you for posting on the Final Fantasy XIV Technical Support Forums.

For your question, Uninstall everything. Go to your library on your PS4 and download from there ARR, Heavensward, and Stormblood. This will prevent you from having to use the disk at all. No need to purchase game again.

For your friends question, Yes they will be able to log into the same account with no problem.

Hope to be of assistance. Thank you for playing Final Fantasy XIV! Hope to see you online.