Before anyone else does what I did and loses a bunch of time setting it up: fava is completely read-only.

This is not true, you can edit single entries from journals or even the whole file from the "Editor" section (it uses Codemirror). Then click "Save" and your changes will be updated on the file in real time.

I use Fava+Beancount and this is not accurate, I use the Fava editor on my beancount files without any problem.