Agreed, 70" isn't bad, but I'd like it a bit lower and would put a smaller chain ring on. I wouldn't mess with the unique rear gear arrangement. The Terns offer a lot more ease of customization and I think will be a lot more fund ride than the Bromptons, but won't fold as small. I haven't ridden either bike, just going by weight, gear range, components and general construction.
Yes, that's how they do it. 44t on the smallest and over 50t on the largest. I haven't ridden a Tern either so I can't tell which is the better ride. I only pointed out that the gear range on Brompton is quite wide and the lowest possible gear is quite low.