Please excuse my long windedness
Unfortunately there is no one definitive way of doing what you want to do. Your overall experience is going to be
affected by your budget and the tradeoffs you're willing to make.
WMC extenders are great for live tv and DVR recordings. They are horrible for playing back ripped blurays. If you encode all your movies into an extender compatible format they will work, but now youre trading off quality and extra effort for compatibility. I don't think I've tried playing my flac files on my 360 or DMA2100, but given that flac isn't natively supported it could go either way. I can try that tonight and let you know.
Extenders also don't really access internet streaming. you didn't mention wanting to do it, but its worth pointing out. Only the 360 can access the modern streaming apps like Netflix and HBO Go. and to do so, you have to pay the monthly subscription for their online gaming service.
You'd think then that PCs at each location is the answer but its not that simple either.
When an extender is running WMC it is logged into the host computer running a remote session. Therefore all activity is tied to that computer - specifically DVR scheduling, recording, and playback. When an extender schedules a recording that will show up on the host pc and all other extenders. When a "copy once" channel is recorded the extender can play that back too, given that its tied the computer that recorded it.
Multiple PCs don't work that way. Programs all need to be scheduled and stored on seperate computers. If you tell your bedroom PC to record the AFC Championship game that will only show up on your bedroom tv. Your living room PC will have no record of you wanting to record that. There is some 3rd party software to manage this, but its not a native solution.
What you can do is share the 'Recorded TV" folder from each computer with all the rest of them. Heres the problem there - your cable company will throw a wrench into it. Any program/channels marked "copy once" will be unavailable to the rest of the computers on the network. So if you record Game of Thrones in the living room you won't be able to watch it in your bedroom. The channels marked that way are determined by your cable company. As a rule of thumb Comcast and Verizon only mark premium movie channels like Starz and Showtime. Most of the other providers mark channels like USA, ESPN, AMC, Discovery, etc copy once as well. You can still watch and record at every location, but you can't share the recordings.
A Slingbox on your HTPC could theoretically work, but that means you can only watch one program in all rooms. Your main PC will be tied up "slinging" a program the kitchen or bedroom, so whatever the bedroom is watching the living room will as well.
Plex has numerous parts which can make it kind of confusing. The general strength of Plex is that ist is very good at transcoding in real time - turning a given file that is incompatible on your playback device into something it can play. So if your file has MPEG2 video, and your device does not support MPEG2, it can turn it into H264. If your file has DTS-MA audio, but your device can't play that, it will turn it into something it can play.
-Plex Media Server is a transcoding media server. It is required no matter what else you use to watch the programming. You tell it where all your movie and music files are. It will talk to your playback device or PC on the other end and figure out what kind of file it needs to send it for compatible playback.
There is a DLNA server component. Some devices, like iPads and Rokus have Plex apps. For ones that don't, but support DLNA, you can still use Plex, just without some of that better features. This is what Plex looks like on a DLNA device (this is the Xbox 360):
-Plex Home Theater is an HTPC frontend like Windows Media Center or XBMC. It does not do DVR or live tv (I think).
This is what it looks like, courtesy @lockdown571
-Most Plex apps look very similar to the Plex Home Theater HTPC software. This is it on a Google TV:
Heres my solution for the WMC/Extender/Movie library compatibility conundrum:
In my bedroom I have one WMC Extender and one Google TV box. The extender goes into the GTV box, and then out to the TV. In a singular tv input and interface I get access to all my recordings, Netflix, and all my bluray rips. my GTV has pretty good file support, but for things it can't play Plex changes it on the fly:
You may also want to look into a Bay Trail nuc, a brand new platform based on the newest Intel quad core Atom processor. I'm very interested in one of these for myself.