Hi Ed,
1) Since you're using MSBPRE it might work to just shift in 8 bits (try it) since that will give you the eight high bits of the result but I don't know what the ADC thinks about having the transfer interrupted in the middle. Personally I'd read the 12bit result and then shift it, EightBit = TwelveBit >> 4.

2) If you're not "allowed" to set CMCON it's because the chip you're using doesn't HAVE that register (likely because it doesn't have comparators) so there's no need to set CMCON. By the way, CMCON is configured differently on different PIC's so just because CMCON=7 turns off the comparators on one PIC doesn't mean it necccesarily does so on another, make sure you read the datasheet.

The TRIS registers are used to select if a particular pin/port should be an input (TRIS.x=1) or an output (TRIS.x=0) it has nothing to with the analog functions per se. The ADC is usually controlled thru the ADCON register but it depends on the particullar PIC you're using, make sure you read the datasheet.

Finally, neither CMCON, TRIS or ADCON are commands - they are registers in the PIC which you assign values to in order to make the chip operate the way you want.

/Henrik.