Item #2 is regarding compliance with 215.2(A)(1), as pictured in the OP. What in 110.14(C) makes you believe you have to use 75?C rating for 215.2 compliance when the conductor is 90?C rated?
110.14(C)(1) (effectively) says that for a 75?C termination rating, you should use conductors rated 75?C, or conductors with higher temperature ratings, provided that the ampacity of such conductors
does not exceed the 75?C ampacity of the conductor size used.
The 75?C ampacity of 2 sets of 400mcm Cu is only 670A, but 215.2(A)(1) requires the minimum conductor size, before any adjustment or correction factor, to have an ampacity of 750, in this case.
110.14(C) permits the conductors with higher temperature ratings than the termination to be used for
ampacity adjustment, correction, or both. As the minimum conductor size requirement from 215.2(A)(1) is being done
before any adjustment or correction factor, it should be based on the 75?C rating.
The Annex Example D3(a) reflects this approach. It has a total load of 99,000VA (119A at 480V). The minimum conductor size based on 125% of continuous plus 100% of non-continuous must have an ampacity of not less than 136A. They tell us that this is
1/0AWG. (Based on 90?C, the min. conductor would be #1AWG.) The example then goes on to apply the adjustment and correction factors to the 90?C ampacity of the XHHW-2 conductors that they are planning to use.