About leg speed, that is what I think will stop her running under 49 at all. Very difficult to run 48 off aerobic power.
1:53 is very much possible, certainly. If she does end up running something unreasonably fast such as 1:51 or quicker then maybe the case for an unfair advantage will be able to be made by the IAAF to the CAS about Semenya specifically.
But you are right, it is all speculation right now so we will see.