You're making it a little harder than it needs to be. I'll address your points, one by one...
Right now, I have a PC with a 150 GB hard drive that houses all my music. I like the idea of adding the macbook and using it as another computer to run MC11. Based on earlier emails, I would likely leave my music on the PC hard drive and access it on the macbook through my home network.
(1) From what I have read so far here and on the net, should I be able to access the music on the Mac using either Parallels or BootCamp? Or, since the music is stored on a FAT32 hard drive, do I need to install MacDrive on the Macbook in order for the Mac to play the files on the FAT32 hard drive?
Macs can, by default, both
read and write to FAT32 partitions. They can also
read-only NTFS partitions. No additional software is needed. However...
This applies only to local partitions (meaning partitions on drives that are directly connected to the Mac via SATA, ATA, USB, FireWire, or some other direct means). When you access files over a network, the computers involved don't really care what the filesystem on the actual drive is, as long as it is shared via a method that they understand. Windows shares files using the SMB protocol, which Mac's handle perfectly well. To share files over a network from the Mac to a Windows box, you just need to turn on Windows file sharing on the Mac (and put the files in a shared folder). The computer connecting over the network doesn't know or care what the file system of the actual shared drive is. Macs can also see and use Windows file shares without any additional software. You can just click on
Go --> Connect to Server (from inside the finder) and type in smb://<windows-machine's-name-here> and click
Connect (or use any of the other network browsing tools).
On the Mac,
you can download the free SharePoints app to define your own shared folders (by default it only shares the "Public" folder in your account's home folder. Sharepoints lets you specify your own folders to share via AFP (Apple's Network sharing system for Macs) or SMB (Windows version of the same).
(2) If I then format the mac's hard drive with HFS Plus, do I need to install MacDrive on my PC to be able to play this music on the PC MC11 program?
The hard drive in the Mac will be formatted to HFS+. That's the default file system for OSX. The only time you need to use MacDrive (which is a Windows-only application) is to access the music from within either BootCamp (which reboots the Mac into Windows XP), or from within the Parallels "virtual machine" (which loads Windows XP in a window inside OSX). While Macs can read NTFS and read/write FAT32/16, Windows isn't so friendly. MacDrive allows Windows to read/write a locally connected HFS partition. Also, when I say "use" MacDrive, you don't really need to do anything once it's installed. It's kinda just a driver. The only time you really "use" the application is to format a local disc to HFS from within Windows. This is rarely needed though because you could just use OSX to do the formatting (which is quicker and easier).
What BootCamp does when you install it is it dynamically resizes the HFS+ partition on your Mac's hard drive to give you some free space. Then, during the Windows install process you can format this space as NTFS or FAT as usual. When you are booted to OSX, it can see both partitions (it's own and the new Windows one). However, without MacDrive installed on the "Windows Side", when you are booted to Windows it can only see it's own partition (it will "see" the mac HFS partition, but it won't be able to read/write it, and will probably report it as unformatted space). With MacDrive installed, then Windows will see it and assign it a drive letter.
The same thing applies to Parallels. Instead of forcing you to reboot to Windows, Parallels lets you run Windows inside a "window" from within OSX. It's still regular old windows though. Instead of formatting an actual partition though, Parallels tricks Windows into thinking a special (large) file on the Mac's hard drive is it's disk (and it formats that file and sees it as it's drive). Unless you install MacDrive on THAT windows install, then that "drive" is all the Parallels version of Windows will see (once you do install MacDrive, then it will "see" the rest of the Mac's hard drive).
(2.5) is it actually possible to create a partition on the Mac's hard drive that uses the FAT32 format, maybe losing some of the benefits of the HFS Plus in order to gain the ease of playing the music easily on my PC?
Yes. OSX can read/write and create FAT32 partitions. The biggest limitation of these partitions is file size though (no files can be bigger than 2 GB). That's why I install MacDrive on all of my Windows machines and format my external drives to HFS+ (usually anyway). HFS is a much better file system (it doesn't have those size limits, it's journaled, and it doesn't fragment easily).
(3) If, following glynor's example, I then hooked up an external HD to the Mac and formatted it using HFS Plus, would I then need to install MacDrive on the Mac in order for MC11 to play the files (since technically, on the mac the MC11 software is running thru Windows XP)?
Yes. However, you'd only install MacDrive into the "windows side" of the Mac, be it BootCamp or Parallels or both. Again, MacDrive is a Windows application. While technically, I believe MacDrive's license is per-computer, they don't prevent you from installing it multiple times in any way. I don't know how it works on multiple physical machines, but I'm sure you're fine installing it on the same machine in both Parallels and Bootcamp (if you do both).
Keep in mind one important thing. Anything you save from inside Parallels (unless you save it to a real FAT32 partition or you use MacDrive) will be inaccessable from basically anywhere else. That's because it isn't using a "real" partition but that special file I mentioned earlier. I don't think there's any way to "open" this file up and "see" the stuff inside it from within OSX or "real" windows. I could be wrong on this, as I haven't tried it (you might be able to load it from inside OSX)....