I'll chime in. Can't address many of questions, and probably none well. Nonetheless, I may be able to help.
I actually use mutliple Mac Minis. All run have Windows 7 64bit and no OSX, or bootcamp. If the machine is going to be home theater use, I would recommend Windows 7 or 8 only. OSX just doesn't have a lot to offer in regards to HTPC and what it has, is limited. I suspect you're not going to be doing general PC tasks on it other than maybe look at videos or pictures on the internet, which Windows can handle just fine.
Windows is a good option because it does support HD audio through HDMI, and I believe the Minis started supported HDMI audio starting in 2011. Granted, there is no HD audio for OTA TV, but if you're blue rays or MKVs, it's possible to have HD audio. On the same note, OSX does not support blue ray videos; there is only one blue ray video and last I read, was still a mixed experience. VLC supports it, but not exactly a WAF friendly. It should be noted, OSX does support blue rays for data. Windows does have blue ray players, but I'm not fan of them because it's possible they'll require expensive software upgrades if there is a significant enough change. I find AnyDVD HD with Media Player Classic is sufficient for the occasional disc playback.
As for the OS, you may want to go Windows 7 over Windows 8 because Media Center is included, which I think is a very good DVR/PVR. And since it sounds like you're in the US, it's the only DVR software, that I know of, that supports cable cards, which certain models of the HDHomerun series also support. Windows 8 does have media center, but it's an $10 fee; I believe this is per machine.
Also, Metro in Windows 8, for whatever reason, I found just didn't work well in the home theater environment. It's not exactly a 10' interface, but given it's tiled based, it would be close enough that it would be servicable once you set if the tiles for the particular software one would use. I was rather disappointed in that regard. I did like it had a smaller footprint and was a bit more efficient.
In addition, check your OSX installation disc, or whatever you were given by Apple. It will should have some directory for bootcamp. This is where are the Windows drivers will be. They may even include a program to install all the drivers for you was well. Trying to do search and install the drivers manual from it can be confusing. I know in my experience for the late 2009 model, there was multiple drivers for a certain hardware. The bad part, Apple doesn't go out of its way to support the Windows drivers, so some stuff that isn't support through Windows Update may have little to no updates. I found this the case for NVidia GPU and install the drivers directly from NVidia.
Lastly, I agree with trying to get a mini with as many cores as possible. Transcoding video is a CPU intense task; it just doesn't use the GPU; that's for displaying video to an attached monitor. Thus, four cores is better than two, eight is better than four. If you're going to transcoding as well, you may want to bump your RAM to eight gigs over the standard four. I don't think you'll see significant increase if you go over eight for the cost.
On a side note, you're going to want to disc spinning HDD drive for storing your media. SSD drives will work, but they're just not designed for the kind of work media storage and retrieval. Quite a few people will have two drives--an SSD for the OS and frequently use applications, HDD for digitized media storage. I wouldn't even worry about an optical drive. There are plenty of extrenal USB ones that work well; I have two blue rays ones, an ASUS and a Buffalo, which cost about $75, that I use to rip blu ray discs.
Personally, I use two late 2009 mac minis. One I have attached to my 55" HDTV in the family room which is use for media playback. The other is in the office which is use for general pc tasks, but also does all my DVR records and media servers; I use Plex and it's server. I should upgrade this machine to something in the eight core area, but I really like the extremely small physical footprint.