About the evolution of flying therapsids/mammals ITTL; I'd say that there's a reasonable chance that some kind of flying animal would evolve from a branch of pre-mammalian therapsids, rather than from true mammals like TTL's (equivallent of) symmetrodonts or triconodonts.
Remember that there won't be any competition from pterosaurs, or indeed any archosaurs ITTL, so it's entirely possible that some branch of arboreal therapsids evolves a gliding membrane and eventually true flight rather early on - possibly even around the same time as the OTL pterosaurs evolved.
However, these flying therapsids would propably be very different from bats, primairily because they would lack echolocation and the well-developed sense of hearing of true mammals.
And unlike OTL bats, these flying therapsids would not need to adapt to a nocturnal lifestyle, as there are no other flying vertebrates to compete with, so their sense of vision would be their primary sense, and they would almost certainly develop large eyes and colour vision.
And the design of their wings could also be very different from that of bats; instead of four fingers supporting the flight skin, we could see a design in which only an elongated and overdeveloped fifth finger supports the flight skin, just like in the OTL pterosaurs. And it's also possible that, instead of being supported by the fifth finger, the flight membrane is supported by an overdeveloped bony "spike" protruding from the wrist. Certain OTL flying squirrels have such "spikes" in order to enlarge the surface of their flight membrane, so such an adaptation wouldn't be unprecedented in evolution.
So in all, these hypothetical flying therapsids might look more like the OTL pterosaurs than the OTL bats.
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However; there is no reason why only one branch of therapsids/mammals would evolve true flight ITTL.
And regarding the fact that it is very likely that mammals or a very close analogue to them are still going to evolve ITTL, there's still is a good chance that some small nocturnal gliding mammal with a well-developed sense of hearing and possibly even a primitive sense of echolocation (there are, after all, also a few species of shrew that use a primitive form of echolocation) eventually evolves, and subsequently evolves into a nocturnal flying species that is very similar to the OTL bats.
Competition with diurnal flying therapsids could very well have a similar effect on the evolution of flying mammals as competition from diurnal birds had in OTL.