1. Not necessarily. Depends on the source/DAC/CD.
2. Yes, the MCD550 has a built-in volume control that you can use.
3. The function of a preamp is to provide additional voltage gain that may not be available from the source (to better drive an amplifier) and to provide a volume control. Additionally, the preamp will add its own sonic signature, which may or may not be what you want. Generally, a lower priced solid-state DAC will usually need a preamp to make the sound less digital. In your case, you’re going to mate a McIntosh CD to a McIntosh tube amp. You may not need a preamp to help with sonic signature.
4. See answer to #3 above. A nice Class A or tube preamp can help the sound be more natural for a very solid-state source and solid-state amp. However, since your dealing with a Mac CD player and a tube Mac amp, I don’t think you’ll need additional preamp.
5. I would start a thread in the preamp forum tittled "Recommend preamp for MCD275". Lots and lots of opinions. I am not a McIntosh expert here. However, C22 is a very old vintage Mac product that probably needs work - replace electrolytic caps, probably replace all volume/level potentiometers. The C2500 is much newer, but I’m sure more expensive.
6. Most likely the DAC in a preamp will not be as good as your MCD550. Shared power supply and less amount of circuits for analog stages, DAC voltage, etc.
7. Your MCD550 can be used as a DAC because has multiple digital inputs (coaxial, toslink, usb). You can connect any digital player, media streamer, computer to it. MCD550 will support PCM audio up to 24/192.
8. This is entirely personal preference.