From your documents, it is clear how to customize HF exchange fraction in a functional where exchange and correlation are separated. For example for BLYP, I have
I don’t think so. Specifying x_hf together with xc_functional will result in a warning/error message.
The only option so far is to not use xc_functional and instead specify the x and c components manually.
I would assume this is possible for all pre-defined xc_functionals, but not sure.
For B3LYP it is not problem.
I wrote the dictionary for it below. The input also compares against the reference, so it should be OK I think. (Verify with some larger calculations when you use it for research) Here you can increase HF exchange to make B3LYP* or other variants (like different VWN).