Hi dinasset,
Some answers to your questions :
1. Tom Keil has not been here for years now. I've already tried to contact him, without success. I'm trying to keep his filters working, but it's only basic maintenance. I won't be able to optimize them (even if that could be probably possible!). It's already a miracle those filters are still working

2. About the faves : you can already rename a filter by double-clicking on his name. There are no plans to allow creating subfolders in Faves. If you want this level of customization, then I suggest you write your personnal '.gmic' file. It will allows to create your own tree of filters, as well as sort them alphabetically.
3. For a "clear start" of the plug-in, this is already possible (and easy). In GIMP, you need to select "Filters / Reset all filters". Then, you can run G'MIC again in its 'original' state.
I will also check the non-working filters in Testing/, thanks for reporting !