Even that doesn’t really make sense to me, unless they’ve done it in a way where everything has to move at once.

Everywhere I’ve worked, if a migration is causing this much downtime then you kill the migration or slow it down. If every change has a 10% chance of bringing the site down, you only do a change every week or two until you can work out the kinks.

...or you keep fighting forward with the migration, because if it's seen as a failure then some pretty big heads will have to roll...

I mean, they are seemingly breaking every week or two so that might be what they are doing.