I'm surprised noonne helped out! :-/
Should be fairly straightforward:
1. In Room A, setup the router...
2. In room B, attach Roamio to coax connection. Turn on Moca Networking in the options of the Roamio. It should get an IP address, and complete functionality (w/cablecard o'course) should be available to you.
2. In room C, you would need a Moca adapter. Like this one: https://tivo.com/shop/detail/moca
Thier shipping is usually way too high tho... shop around!
. Attach put the adapter between the Tivo and the wall and attach cat 5 from the adapter to the tivo. Should just "work" at this point, tivo should get an IP address, all functionality should be available.
3. In room D, by access point... if you mean wired: Drop in a second Moca adapter. (You can get a 2-pack for $10 off). If you meant wireless access point, add your device of choice (wap, wireless router) to the adapter, or use a Moca router configured to be an access point.
4. Be aware of the frequency range of any splitter(s) that are in the house's wiring. If the splitters are old or cheap and dont pass through a high enough frequency range, the Moca signal may be attenuated.