I know what Hanlon's razor is, and I can guarantee you FB Engineering knows what a rollback is and how to know whether you need one or not. I'm not sure where you're getting the idea that they're that stupid.

Would applying such a 'rollback' involve replacing facebook.com email addresses with the original external domain addresses in the contacts list on the devices affected by the write-over issue?

If so: Facebook have taken private data and placed it in a central location

If not: it isn't a rollback that I understand

Disclaimer: I'm not a programmer

Yes, a (hypothetical) "complete rollback" would involve replacing removed data. Sure, data privacy would be a concern, but it's beside the point that rolling this back would necessitate un-overwriting.

how about 'the evidence on display in front of us'?

Hanlon's razor is not a law of nature.

Neither is the opposite.