Are the two middle distance races for M/F21s considered separate races for ranking purposes?
Yes, they would be ranked as separate races, and both will be ranked as long as there are standard interval starts.
How are the starts being done for the Middle Final? Chase start, reverse order, random?
The starts for the middle final will be in reverse order (slowest time first, fastest time last) with 2 minute start intervals. In case it was unclear, both the B and the A final run the same course.
Does this mean that all the B final people run before the A final?
Yes, that's correct. There will be an interval (undecided at present - probably 15-30 minutes) between the last B final start and the first A final start.