Your request is not unreasonable. The agent is correct that UA is getting very stringent on Premier Zone seats for passengers traveling on 2 separate records. With regards to exit row seats, really you can forget it. If they are not on the same PNR then they will not receive exit row seating prior to checkin.
With regard to the 500 mile certs: When a Premier level passenger is travelling with a non-Premier on a separate PNR, they can waitlist for the upgrade and if the upgrade clears, they are allowed to bring one non-Premier into the upgraded class of service with them ***as long as another upgrade seat is immediately available when the Premier calls in***. If one is not, then the Premier can actively waitlist their non Premier partner.
das- If you don't like then answer you get, hang up and call the 1P number again. usually, unless your request is absurd someone will eventualy see your point. If it turns into an argument the agent is likely going to document what you have been advised in your record and you have just worked against yourself. That's a whole other thread...