1a. Increase the resolution with Overscan prefs, if your monitor can show any more.
1b. Use Interlaced modes if you can tolerate them.
1c. Increasing colours beyond 256 is not on.
2a. No idea, but if the monitor can sync down to 15KHz then who cares what sort of plug it uses?
2b. Use Doubled screen modes, and quite a lot of monitors might work - but you will likely not be able to play some games that way as many have a habit of forcing 15KHz modes.