Is it obligatory for someone who intended to perform Hajj from Ta'if and went to Jeddah to join a Hajj campaign, to return to the Miqat of As-Sail Al-Kabeer for Ihram, or is it permissible for him to enter Ihram from the Miqat of Rabigh?

The best is for you to enter into ihram from your original miqat (As-Sail Al-Kabeer) before going to Jeddah. If you pass the miqat without entering into ihram, then it is best for you to return to it. However, if you enter into ihram from Rabigh, there is no harm in that, because it is further from Mecca than the miqat of As-Sail Al-Kabeer. It is permissible to enter into ihram from another miqat that is the same distance or further than the original miqat.