This all being a perfect example to agama's other question about whether to update or not - calibre changes very frequently, and you are looking at code that Kovid rewrote recently to support tweaking things other than ePub. So you are tiptoeing on quicksand in this case
Re the .ui files thing - from a calibre plugin perspective I have always avoided them. When I first started writing plugins there were question marks over compiling code in plugins etc that were never completely answered at the time. The Qt designer is not sufficiently more productive to have ever got me bothered about revisiting it. So every plugin dialog I have written just has handwritten dialog code, similar to the net effect of using the designer, then compiling the code, then including that in your plugin, I find it faster for my purposes anyways.
In the calibre code you will find a mixture - some dialogs are handcoded in the same way, whereas the majority of the calibre gui uses those UI files. My advice would be to not bother with adding another learning curve of the Qt Designer especially given your plugin doesn't even really need a UI from what you have described, but that's just my opinion