I think that 'fast subset' is the most important color changer to have a default shortcut for, which we do currently.
I'm against using
Insert for the inking tool; I think that inserting new layers is a more prominent usage than the ink tool. Same reasoning as before: the current Ctrl+PageDown|Up makes it seem like inserting a new layer should be harder than inserting a new point, which seems dubious (almost everyone will add layers at some point, not everyone will even use the inking tool).
I think also that only one of 'Insert painting layer (above | below) current layer' should be shortcutted; Perhaps I am wrong, but I expect that most users want to insert layer above the current one 80+% of the time, and we would be more considerate of them to give them one shortcut and expect them to move the layer manually in the 20% case, than to give them two to remember (of which the second is likely to be hard to remember because of how rarely it is used)
Mysteriously, I agree with you : a (key, ctrl+other_key) combo for related actions is difficult to remember. Adjacent keys with the same modifier is best IMO (eg [z, a], or [ctrl+z, ctrl+a])