It sucks for sure to potentially lose out on MR points but it sounds like AMEX has made the decision for you regarding an ongoing relationship. You and AMEX both have the right to go your separate ways but, as a business, it is a best practice to keep a good customer happy so that they continue to patronize your business.

A 700 score is not that low. However, to open a platinum account one may need a higher score and AMEX may consider the restoration similarly as they would a new account. Personally, not even considering your history with them as well as the likelihood(if they believe you) that the cause of the cancellation was out of your control would be more concerning to me. You shouldn't have to jump through hoops for them given the state of the credit game.

In my opinion, I'd focus more of my time on an attempt to rescue your orphaned points. Of course, you would rather have the flexibility to transfer to whomever best suits your agenda in the future but current events may dictate that a final home be selected now.

Best of luck to you.