I have a similar setup. I absolutely love it but the Mrs is a different story. If you have all your HTPC's record to a shared drive then all HTPCs will have access to everyone's recordings in a single location. The best solution is WMC extenders but they have their limitations as well. Take a look at the Ceton echo.
http://www.newegg.com/Product/Product.aspx?Item=N82E16815706003
You just add those to your TV and stream from a main HTPC with enough tuners. If you already have the HDHRP then add a Centon intinitv 4/6 dedicated to the main HTPC and you'll never run out of tuners. You'll need to make sure the server HTPC has plenty of cores and ram. My main HTPC has an PHII x6, but did not go the extender route.
I decided instead to go with HTPCs at every TV because of browser limitations with extenders and settop solutions primarily due to Silverlight with premium cable streaming and a difficult time handling large libraries.
Be warned Windows commitment to media center dev seems to have ceased. The WMC DVR functionality is inferior to DVRs from a decent cable company or DTV.
To answer your questions;
1. HDHomerun Prime is an awesome device you'll be happy, and able to watch live TV on any computer.
2. Other computers will just need to have access to a shared folder to access recordings.
3. You can watch recordings in windows media player, you don't have to do it through media center.
4. You can run MCEBuddy in the background to remove commencials and convert your recordings to a portable format (MP4,M4V) so you can watch on something like an iPad offline.
5. Building a more capable main HTPC will let that act as a server as well
6. For basic HTPCs you'll be pleasantly surprised how little horsepower you need for flawless 1080p content. Just built one this week for $300 with a quad core and ssd.
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