Is it legislated to shorten prayers in Al-Qunfudhah for a resident there whose home in Jeddah is 350 km away, and he visits it on Thursdays and Fridays? And is it permissible to assume ihram for Umrah from Jeddah?

If you intend to perform 'Umrah from Jeddah, then perform ihram from there. If you intend to perform it from Al-Qunfudhah, then perform ihram from the miqat of the people of Yemen (As-Sa'diyah). It is not permissible to shorten prayers in your hometown because it is your homeland, and you must pray four rak'ahs. Likewise, in Jeddah, you must pray four rak'ahs according to the majority of scholars, because you intend to reside there for more than four days.