Interesting developments:
SPMC from the amazon app store
If I can do a bit of research and verify SPMC works on these sticks, I'm getting some Christmas Presents.
XBMC does need to be sideloaded on FireTV, SPMC does too. It's in both Amazon's and Google's app store.
I don't have SPMC, but the fact that the app updates is double edged - Updates could break or fix things. You'll be tech support. SPMC is supposed to include optimizations and remove unneeded options. I think putting it on FireTV would be best because then your FIL can do other things with the device.
Edit: they both need to be side loaded!
When I go to the SPMC link the store tells me that SPMC is not compatible with my FireTV. I have side-loaded XBMC and SPMC but I assume that if I install SPMC from the Amazon store it will show up in my apps list on the AFTV home screen not buried in the all installed apps under settings.