To your last point: Not necessarily, as they are continuously-scaled across grade levels. There may not be, as you rightly point out, enough items (if any) of difficulty level 300 on the 2-5 reading test to obtain a 300 score. Also, as these are Rasch scores, and not deviation scores, comparing scores in one subject area to another is even more sketchy than usual. (NWEA specifically says not to do it.)
I defer to your expertise. I only saw the 300 as an upper number on some publication relating to MAP. In reality, the reading scores are always lower across the board compared to the math scores for the same percentiles. That's why I can only vouch for the possibility of a 280 on the math and a 250 on the reading for the MAP 2-5.
