I don’t know what you mean by “backwards compatible”, but…
That’s correct. If you configure the things in a text file, then they can’t be configured in the UI, and you can’t even send any configuration to the thing. This will mean that you cannot configure devices - including, for example, Z-Wave device parameters.
No - if you configure a device using text files, they you can’t configure it in the UI. If it was discovered automatically, then so long as you don’t start configuring it in the text file, you can configure it in the UI.
Basically, you have to choose how you configure your things - either text files, OR the UI. You can choose differently for each thing, items, and rules, but if you configure “something” using a text file, then it’s fixed by that file. This does mean that some things can be configured in text files, and some in the UI - although I’d probably not recommend this as it’s probably a recipe for confusion and doom.
For this reason, I personally use and recommend configuring in the UI for ZWave at least - otherwise there’s no way to configure devices.
(I hope that makes sense).