Hot water radiator valve leaks

I haven't had very good luck with repairing steam radiator patches from the outside (sample size: 2). Steam under pressure is very good at finding microscopic holes between patch and substrate, and enlarging them.

What I have done successfully is an inside patch with epoxy, using a wire brush and steel wool to prep the surface. Getting at the leak may be a challenge, particularly with a radiator that has more than 4 sections you may need to break the sections apart (if you do, this is an excellent time to get the exterior sandblasted and painted).

Simplest and most cost-effective solution is probably to find an architectural salvage shop and replace the radiator (they might take your old radiator as a partial trade).

I'm afraid your super is right. The supply valve must either be fully open or fully closed. If it is partially open, condensate cannot properly drain back to the boiler. It will collect in your radiator instead. How much heat is put out is regulated by the size of the vent valve. If the boiler works by central control, meaning the steam goes on and off for the whole building to regulate heat, you can regulate the heat put out by the radiator by getting a thermostatically controlled one pipe steam radiator vent valve, or simply getting a smaller vent valve.