You sound more like an "anti-maintenance" man yow!
Some have had some luck with the MFJ artificial ground, but I think you need to give up trying stop-gap measures with an unbalanced antenna, like quite a few others have suggested.
Although I wonder about the life-span of coax in a dry vent
, you could just run another one up and install a trapped or fan dipole for the higher frequency bands that you want. That would you give you much less loss in your coax run and should be a much more enjoyable antenna for you and the tenants.
Whatever you do, it would be a very good idea to install current chokes at the feedpoint before the coax runs into the building to block rf from running down the outside of your coax.
Any chance of getting a real ground? Maybe you can convince the owners of the advantages of a good earth grounding system for the building against lightning strikes and make sure the line runs right by your apartment
You're kind of stuck in a hard spot... top floor and you could run a balanced line out the window (300ohm twin lead would be pretty stealth since it looks kind of like coax), bottom floor you could get a real ground easily. Maybe you can switch apartments?
Good luck and keep asking questions. Someone may have a charger for their oxygen, pacemaker or insulin pump on one of those outlets some day and that wouldn't be a good thing to have on your conscience if there was a problem due to that