Welcome Nils, and thank you very much for taking the time to write all this! Very nice works on your site — I am very eager to see what people like you are going to be able to do with bella.
It is interesting what you say about the need for light quality, even (or especially) in preliminary pre-viz. We had debated internally whether it was really worth the time for us to add a physical sky model, since our perception has been that while quick & easy, sky simulation has often been one of the more identifiably-CG aspects of renderings, and that production has moved toward heavier reliance on HDRI in most cases. So your use case is interesting — and thankfully we do not need to choose, as we have indeed been working (are working, presently) on adding a nice physical sky & sun environment.
On materials, that is a much deeper question. It almost sounds to me like there would be a place in your workflow for a dedicated tool for translating materials, in an automated way, and not necessarily using functions actually inside the plugin.
Of course a plugin must provide integrated ways of working directly with bella materials, and for translating native materials to bella, but plugin-based functions for dealing with external materials can often be obscure, fiddly, out-of-the-way types of affairs (i.e. click button X in toolbar Y to bring up the weird interface for this seldom-used function), when given what you say, it may be more interesting to look into having a dedicated and efficient standalone tool for converting or generating entire libraries of materials, which are subsequently used as-is from within the actual modeling environment.
Regarding mxm specifically, since it is a proprietary binary format, the best we could do would be a python script that a licensed maxwell user would be able to run in their copy of pymaxwell. However, something like that could be integrated into a standalone tool like I mention above.
All that said, though I am still designing the way that we will reference external bella materials (i.e. there is not currently support for it), we start from a good place with the bella .bsz format, which bundles a bella file with any resources it uses. Furthermore, a bella “material” file will really just be a bella file, so it will also be possible to support having more than one material in a file, or have a file that contains some geometry along with its materials, and so forth.