Hi!
Before answering the first question, the rule is that you can use a CG, but that means you can't use a sprite and a background anymore (since a CG is counted as background+character).
But yes even so you can still have a main menu solid background with lineart (from the CG) as a form of alternate menu, that specific graphics would also be within the rules. But it has to be the lineart from the CG, not a separate sprite line art made specifically for the menu.
The second question, the itch.io cover does not count for the jam limits, that can be anything.