I was prepared to say that as long as you have actual pay grades you were safe from a retaliation claim, but with that huge a discrepancy I'm not so sure. She's wrong, btw, that ANY reduction in pay would be considered retaliatory since you do have actual pay grades with actual ranges but even if you won the retaliation claim (and I'm not saying you wouldn't eventually) you'd still have to defend it, and I have a feeling, from what you're telling me, that you might end up with some serious problems before you were through. You're already on the hook for one massive mis-calculation; truly, whoever determined that this job fell under the Learned Professional exemption did not have a CLUE what they were doing. That's a big enough mistake so that it's almost certain that you've got other mis-classifications in there as well.
I trust that you have corporate counsel? You need to involve them here. You might want to see about splitting the difference but this babe has you over a barrel and you're going to have yourself a time finding a way out of it.