If I have almost 100 knobs (in different panels) that should all look the same, a global LAF is better, isn't it?

If they all look the same and all have the same colours then it doesn't matter. I'd still go with local so that I had control over individual knobs and so it didn't make the HISE UI knobs look weird, but it really doesn't matter.

With a local LAF, I would need an additional 5-10 scripts for each knob for the individual LAF.

You can use the same local LAF object for multiple controls. I put almost all my laf in a single file. See this as an example.

2. all declarations in one script?
   Does it make sense to collect all component declarations in an external script "declarations.js" or better in different scripts at the beginning of the respective function?

As I mentioned I put almost all my laf functions in a single file. I put component declarations into separate files, each with their own namespace. So if I have a set of controls all relating to a mic mixer, then they go in Mixer.js with the namespace Mixer.