While on ground, during cockpit preparation, with the INT/RAD switch in the NEUTRAL position and use of no electronics, are the cockpit crew members able to hear each other clearly?
You are only sitting a couple of feet away from the other chap/chapess so it's not a problem unless you've got your headset on both ears and have something like the ATIS on really loud in the background.
What do the selection and rotation of the INT and CAB reception knobs enable?
the turnable knobs on the ACP enable you to select the source and volume for the headset earphones and speakers.
i.e. during normal ops you'll have VHF1 (ATC), VHF2 (possibly 121.5) and INT selected. If the cabin crew call you then you select ATT (to speak) and CAB(to hear) and turn the volume of any of the other ones down as required so that you can comfortably hear the cabin crew over the other transmissions.