Thanks all for the continued feedback. I love to learn about mechanical things and building one's own bike is truly gratifying. It'd definitely easier than wrenching on my old Ducati. I have a few more pics to show a different view to add to the chain length/shifter discussion.
But first,
Ryan: My Sam had the BB already chased to my relief as I did not have the tools to do it. I do not know if that was done at the builder or by Riv. I do know Riv opened my box as they packed the derailer in it along with a few other parts. The BB was not faced, and I see no need to do that with modern sealed BBs. There is only cartridge to BB contact on the right/drive side. There is so much threading on both ends to square the cartridge that I would much rather have the paint in there to protect the metal than attempt a home-made facing. I've seen videos of builders taking a box cutter to the paint on the BB and calling it facing. Terrible idea. Just make sure you use plenty of (pick your favorite) lube in there. I generally went with anti-seize for parts that I would not generally move again (like the BB), grease for frequently moved parts, and blue thread locker for fiddly parts that I did not see adjusting anytime soon.
The Nexave. I can see why the idea of a Rapid Rise or "Low Normal" never really caught on. The installers probably hated it. Keep in mind that the default (slacked cable position) is the low (first) gear, lining the pulleys up with the largest cog on the cassette. So when installing a new chain, even if you sized it right on the largest cogs and adding two links, you still have a wresting match on your hands when you thread it through the derailer pulleys as the darned thing does not want to move to the smaller cogs (providing slack) unless you grab the derailer and pull on it. Your chain is not connected at this point, so you can't turn the crank and shift to a higher gear, thus providing the slack. So, one's slack, if needed, must come from using the smaller front derailer chainring, or a handy dandy homemade tool that I was supposed to know about. It's a bit of a dance.
Chain length: Here are a few photos with the chain in the most slack/highest gear position. I feel comfortable with the sizing as it falls within the recommendations and seems to work. In friction mode, shifting is quick and quiet. I just need to figure out the indexing, which is only academic as I will likely never use it. Can't leave a challenge alone.
The derailer below is coiled up pretty tight in this gear selection, any more chain slack and it would likely make contact. I've seen Nexave derailers with the Mega Range long pulleys on them, but this isn't one of them. The instructions for mine say it can handle a max 32T, I have a 34T, but it makes the reach.