Really good question... _give me more fossils!!!_ :-)
Leathery? Hah. Pterosaur wings weren't bat wings, they were reinforced by those famous "aktinofibrils" and therefore quite stiff.
Bingo IMHO, that indeed seems to be the way the evolution works.
I don't completely understand this... anyway, there probably were no suitable empty niches at that time, I'd say.
BTW, I've thought once more about the hypothesis that mammals replaced dinosaurs from the bottom up... Dinosaurs never got into certain niches for small animals because they had been filled all the time by synapsids. Examples (in decreasing order of support):
- the rodent niche (dicynodonts?, tritylodontids, haramiyids, multituberculates, gondwanatheres, no dinos);
- various arboreal niches (probably arboreal mammals, like *Henkelotherium*, are known, but so far no dinos; *Microraptor*, well... :-| needless to say, birds got into the trees)
- The Really Small Insectivore niche (but neck vertebrae of a 20 cm long coelophysoid have long been known from Nova Scotia, and birds got into that niche);
- the platypus niche (occupied by platypuses since the Early Cretaceous).
But AFAIK no suspicious increase in size is known from end-Cretaceous mammals (the largest Mesozoic mammals I know are from the Early Cretaceous, *Gobiconodon* [40 cm] and *Kollikodon* [?1 m]), neither did mammals appear in niches that had so far been held by dinos.
I strongly suspect there are people Out There who know more about Mesozoic mammals...