You can go both ways, shouldn’t matter.
Fairphone OS uses the stock Android recovery and under certain circumstances is able to restore that over a TWRP installed by the user.
Fairphone Open OS uses TWRP as recovery itself, so it shouldn’t happen with Fairphone Open OS.
That’s not a bad idea, but not strictly required. If your phone is encrypted, you need a halfway current version of TWRP that supports encryption.
TWRP can update itself. If you want to select the TWRP image file in TWRP, there’s a button to switch the file view from displaying ZIP files to displaying IMG files.
Excellent idea 
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