I have the Leonardo as well and the 1 button navigation works well. Setting nitrox is easy and can be done fast.
Leonardo has no 21% nitrox. Other computers have air and ean21, because ean21 does calculate oxygen toxity. Some computers are blocking you out, if you switch from air to nitrox withing a certain time. That's why you dive ean21 on the first dive and ean32(or whatever) on the next dive.
You're right, it goes from "air" to 22. I haven't actually checked this before, or did and forgot, because I only dive normoxic nitrox (with thanks to @EFX
According to the Fine Manual the Leo tracks PO2 and oxtox regardless of the mix. Personally I think not doing that would be a dumb design decision and I wouldn't recommend a computer, or anything, designed by stupid.