It’s at least REST-ish! Client-server, resource URIs, stateless. Checks the REST boxes, I think.
The Python PlexAPI docs are a great way to get a sense of what’s possible, even if you don’t know Python. Some of the examples are only a few lines long and you can build from there.
Yes you can edit data. I tested setting/clearing/locking a number of fields in Python.
I started to explain how to do it with HTTP, but then I found this - and it’s already formatted and nice and clear. 
As with anything, the more powerful the tool, the faster things can go wrong. Make a backup, blah blah.
