We have been trying to get back from him for some time now through different channels, but unfortunately he hasn’t been online since early May. Once he is back online we’ll have a better picture on the migration. Our understanding is that message merging from a future imported category to an existing one is possible.
At least 2-3 moderators is something that is believed to be important for a self-sustainable moderation and the intention is to support communities to get there. If this turns out to be impossible, I expect work with communities to accommodate.