> SLS launches are so expensive, and so slow to build and prep to launch, that we cannot fit in a uncrewed mission between 1 and 2 to test or validate fixes or models

If they’d wanted to they could have launched an empty Orion crew module into LEO on another, cheaper, rocket and tested re-entry. The crew module by itself is less than ten tonnes.

How would they get it up to the required reentry speed for it to be a valid test? They already know the heat shield works for reentry at LEO speeds. That's not where the problems occur.