Is it right to define ground as a pool of surplus free electrons, like a metal?
As usual, electron-based thinking will lead you astray. A metal or other conductor has mobile electrons which are usually referred to as "free", but there is an overall balance of their number with the number of protons in the metal, so they are not a "surplus". The nearest thing to a surplus electron source might be a N-type semiconductor.

Dipoles and monopoles-with-ground are usually considered separately. Dipoles aren't grounded. My antenna theory is rusty so I'm going to refer you to the internet.

The important number is not resistance but impedance at the particular frequency of interest. The resistance of a good antenna is as close to zero as possible.