Is it possible to access, particularly insert into, a database while it's being restored by pg_restore from a custom format. If yes, then should I care about preventing clients from accessing the database while pg_restore is running, or the restore operation is "transactional" so that after it ends all changes made by clients since its start will be lost?
If you want to keep concurrent sessions out of your database during pg_restore, you'll have to block them with a pg_hba.conf entry.
There is no protection from concurrent sessions inserting or otherwise modifying data while pg_restore is running.