Unless I am missing something but dumping your entire db then reimporting would take hours in most cases. Staying available during that time would be a nightmare.
Not literally immutable, because you can change it, but changing it is more difficult and more perilous than starting from scratch.
Wasn't the point of MongoDB was supposed to be that it's all flexible and schemaless? You might ask that. You might not get an answer. Like I said, I never use MongoDB as a distributed system anymore.