>> My question is this: how long does the MAX have to be grounded before the costs of it not being service end up more than (1) the costs of recertifying pilots, and (2) the costs of making the other design changes that would be needed to certifying as a new type...

Not sure how long, but they passed that point a long time ago. From what I've read, one of the goals was to not require pilot training on the new plane, and that is already going to be required now.