Before you get too invested, I would seriously consider whether the NMF scholarship is something your kids would use- it can be a great deal, but only if your kids decide to attend specific schools; many schools ignore it completely. I don’t know, but in the end I felt it didn’t seem to add much to our DDs application, as many (?most) kids applying to the same schools had similar scores.
If your girls have already taken the SAT, then you already have information about how they do with this format (though the PSAT format is somewhat different, our kids didn’t find the differences to be significant).
So I guess I would use those data points to guide you in how much prep you want/expect them to do. Our kids (both untested) luckily do standardized tests very well- both easily cleared the benchmark for NMF taking the PSAT cold, without having done the SAT or ACT. (and one went in with the wrong calculator, so did the math without it, the other had a huge nosebleed and almost didn’t make it to the test center on time...). But we kind of expected them to have an easy time, and our kids abhor test prep, so we just went with that.
If you know your kids have areas that need work, you could consider it, but you have several years before the 11th grade test, and personally I think that just taking the PSAT twice beforehand, as your girls would be doing, should be more than adequate (unless they have test-taking challenges).
Regarding accommodations, we don’t have experience with this, but I think there are several past threads that address this issue. If I remember correctly, it can be a difficult thing to achieve, and scan take a while.
Good luck!
