Licensing for mkgmap styles

I’m confused on license for style files that are used use with mkgmap.

My interest is inspired by the complains I’ve heard about people making profit on commercial maps based on OSM. E.g. AFAIK openmtbmap author stopped to publish his styles because of that (commercial products derived on his styles).
What they violate (or not violate) by charging for maps is a separate question (I know it’s legal in some cases), but I’m interested in styles - because styles makes maps reproducible.

In particular:

  1. mkgmap itself is GPL, meaning that built-in styles are also GPL. I have a (controversial) feeling that ANY custom style files for mkgmap should be GPL because they are essentially ‘derivative work’: the starting point for any custom style is probably the default style. But probably that’s impossible to prove. Or is it?
  2. Generated Garmin maps are ODbL. As I understand, it’s different from GPL in terms of reqrements to release the ‘sources’ - i.e. map producers don’t have to release the styles. Is it correct?
  3. Most important one. Suppose I take the ‘commercial’ map based on OSM (with custom closed style files), and create the style that matches it (produces identical map if used from the same extract), can I do this? And then publish with some open license?
  4. What about a tool that generates such style by kind of automatic reverse-engineering? Side-note: I’m not sure it’s entirely possible technically, the question is about legality. I would be glad if such tool already exists (I’m aware of TYP extractor only).