Stranded vs Solid Copper

 IMO it depends on your system..The more resolving the system the more likely to hear a sonic difference IMO favoring 9nines solid or better...

I assume the other 5 outlets and light are wired with Romex 12/2  if it's a 20amp circuit back to the breaker box. I guess using 4 or 5 feet of stranded at the end of this run might make a sonic difference but I really don't see how. You can use stranded and it is more flexible though it should be terminated at the outlets slightly different than solid to keep the strands tight. 

Not getting the point of a home made box, hospital grade receptacles will fit in standard boxes. Not  sure home made would meet code might want to check on that, if there was an accident insurance companies can get a little testy  on such things. 

Yup, expected or intended losses are excluded. 

The plastic box was buried in the wall so far that the electrician used 2 + inch screws to connect the receptacle to the plastic box. When I plugged my Silnote cord  into the receptacle it pushed the receptacle back into the wall and broke the plastic cover.
i could have mounted the metal box right over the existing box, but I thought this might afford me the chance to gain some flexibility and expand the number of outlets to two.