Keeping in mind I don't use lathe features but do a little development for EMC...
1. yes may be small differences. I understand EMC is pickier. No tool wear offsets though.
3. g71 is not sure if its exactly like fanuc but is based on it.
4. i'm not sure about the c11 board but I would bet yes. according to the c11 web site yes.
5. EMC can configure the parport as 12 outputs and 5 inputs or 4 outputs and 13 inputs
within that restriction the pins can be any supported signal.
6. yes there is a charge pump component - not familiar with it
7. sort of. buttons and display items can be added as panels on the side of the display.
complete customization of the screen is not easily possible.
8. Most definitely yes! people use it on 5 axis multi ton machines for profit.
In your case look at the HTML docs here: EMC2 Documentation - HTML version
This is the latest released version though 2.5 can be downloaded and used it's almost ready to release.
You will most likely use stepconf to build your configuration. So check the docs on that.