Hi,

To get exact answers you (or I) have to read the data sheet for the PIC in question. However you have not stated which pic you want to use.
I can make a few comments though.
1, 12V will damage a PIC if applied to a pin without current limiting. If you limit the current to a milliamp or so the pin will be clamped by the protection diode to just above the supply (Vdd) voltage.
The datasheet will tell you the useable voltage range, to sense 12V you would have to use a resistive divider to drop the voltage to within this range.
2, Check the datasheet some use the supply (Vdd) so it's as good as you make it, other have a "real" reference and they are pretty good, but it depends on your needs.

3, see 2 above.

HTH Robert G8RPI