Right now, I am running tubes for 26x1.9-2.2 in a 26x2.35 tire.
I don't think innertubes are an "exact science". There is seems to be enough elasticity and manufacturing variation in tubes to allow for poetic licensing with tubes. Also, tire sizes seem to vary between manufacturers, and the volume of the void is not exact.
Good question. With my 28mm Spec. Armadillos the 28/35 (or 32 I forget) fit better than I remember the up to 28s working for 28mm Avocets years ago on another bike. Both worked in each case. I do remember not being quite as sure with the earlier tires with the respective tubes. Anyway 28s seem more like "true" 28s now-a-days, I'd say the bigger MAY have the slight advantage, just my impression, it may not matter much really.