First - will this ever work with MythTV light as effectively as a single (headend, backend, PVR, tuner, etc.) "cable" box?
I had originally seen Raspberry Pi as a frontend only. However a number of people are using it for backend as well. I think you will need at least two raspberry Pi's, one for frontend and one for backend, due to the raspberry pi being limited to 1 GB of main memory. You will need a hard drive for storage of your recordings. I don't know how big an SD card works with raspberry pi, but it seems futile to try to use an SD card for recordings.
Second - how about MythTV with a full blown compile/install? If not I'll move on.
I don't know what you are asking. You are welcome to compile and install MythTV on the raspberry pi. Also the mythtv-light is a full blown build with everything in it.
Third - does anyone have a detailed build process that they can share with me if I should proceed?
Look here for raspberry pi setuphttps://www.mythtv.org/wiki/Raspberry_Pi
Look here for build instructions, which do include a section for building on raspberry pihttps://www.mythtv.org/wiki/Build_from_Source
Let me know if there is anything missing or unclear in the documentation.