About Part a) - can you add the ability to set the default as “Checked” for both filters ?
@sauron - not sure why/how this is marked as a solution BUT
Part b) of my question relates specifically to “Constrained VBR” at 99% as shown in the screenshot. The “Default” preset uses “Quality-based VBR” which is not the same. Did you try saving a preset using 99% and the other values depicted? I did and it does not save the value - it reverts back to 59%.