I don’t think they will ever be merged – they serve very different purposes. Chat is ephemeral and is used to have quick discussions with people. PMs are permanent and are therefore used for private discussions that may need to be revisited in the future.
I agree that for many communities, chat simply isn’t necessary and can be turned off. For others (like internal communities) it is important.
No because that wouldn’t make a lot of sense. It makes more sense to go the other way (which is possible).
Yes – you can see that the third one in my list is a group chat.
