Is it permissible for someone who has come to Jeddah - with the intention of visiting it first, then performing Hajj if it becomes feasible - to enter into a state of Ihram from the Miqat for the people of Jeddah, given that he resides in Riyadh and his wife has arrived from Jordan?

If you form the intention of ihram for Hajj or Umrah from Jeddah, then your ihram will be from Jeddah. However, if you intend it from Riyadh, then your ihram will be from the miqat of the people of Riyadh. It is a condition that going to Jeddah is not for the purpose of performing Hajj from there. And whoever is hesitant about performing Hajj after completing their affairs in Jeddah is not obliged to enter ihram from the miqat. If they then decide to perform Hajj, they should enter ihram from the place where they are.