I've said it before: You need to think seriously about crank length, because that's something that becomes ludicrously expensive to change afterwards, but not so much when you get the dealer to do it on a new bike. I guarantee that 170mm cranks are too long for you, that you won't regret 160mm (the smallest size that's easily obtained), and would probably benefit from shorter (but finding out how short is NP-hard, due to all the unobtanium involved). Same goes for stokers, naturally. Crank-shorteners might be a reasonable botch for SmallestCub purposes (I don't see him managing on 170s for a few years yet).
J Random Leggy YACFer would have no problem riding on your 160mm cranks. They might not even notice the difference. It's like saddle height: too much is less forgiving than too little. 10mm shorter cranks raise the gain ratio of your given gearing by what feels like a couple of inches (gear inches are a silly unit that doesn't account for crank length).
Dynamo hub makes good sense for the same sort of reasons. The 60lux (non-'R') Cyo is probably the one to go for given the low fork crown with a 20" wheel. Realistically I don't see you doing the sort of distances that make USB charging worthwhile for a couple of years, and lights are an easy retrofit and always getting better/cheaper. Cyos Just Work ('N' means "has a switch", 'plus' means "has a standlight capacitor"). A dynamo light that fits to the plate on the rear rack wouldn't be a bad idea either, if only because they're always perfectly aligned and tend to have lovely reflectors built in, for absolutely zero faff. OTOH, that's an easy retrofit too.
I reckon the Alfine 11 is your best bet, set up so the lowest gear is somewhere in the low 20" range. It's a tandem, so you'll reach escape velocity pretty quickly on the downhills anyway, and 100" is more than enough for mere mortals to bomb along with the mythical fenland tailwind. Not worth going to Dual-drive (and associated dérailleur-related maintenance) for the higher gears that gives. Far more important to gear it low enough that you can drag a tired Cub plus camping kit up a Bastard Hill.
BB7s aren't even a question.
I'm sure someone can dig out some tatty SPD pedals for you to keep in the bits box, so cleated volunteer stokers (or indeed captains) can get a spanner out if it matters to them. Sorted. Might be worth having some of those
partial toeclips on your captain's pedals too. Not enough foot retention for pulling upwards or clipless moments, but should reduce the chances of leg damage from Teethgrinder Ultralite moments.
Tandem top tip: a length of elastic (3mm zipcord sort of thing) run between captain and stoker's pedal platforms on both sides will make the pedals self-righting, for ease of toeclip operation.
