I've read through this thread a couple of times and am very intrigued. I would love to replace the M&S intercom in my house with tablets. So how is it going a year in? Are you happy with it? What software are you running now (home tablet, room tablets, PC)? Can a stranger walk in and use it without a training session?
One thing I missed reading through the whole thread twice is how are you putting the 12V on the old intercom wires? I assume it was originally coming from the intercom "master" that you took out of the kitchen.
I'm not really interested in running a PC for this, but could if it would make things more user friendly and reliable. Though for my purposes I see these more as video intercoms and wall-mount home automation controls than streaming media devices. I do have a MiCasa Verde that I haven't done much with yet, a TabloTV, Sonos, and just ordered a Ring doorbell. All of which I may eventually want to integrate.