I'm trying to make ALL mobs do twice as much damage as they do normally, but I noticed Skeletons and other mobs that wield bows (or crossbows) don't have their damage multiplied. Is there a way around this?

can be ignored as not 100% sure but I do know some gist... granted I am noob too at some things.
it is because it is the arrow or projectile think this is where modifying their held item will come in... forget since been many years but I think their 'Item_slot' was 0 for arrow or projectile... if I recall right the arrow needs to be modified
honestly though... curious about this too... I have had my own questions related to things... fixed the issues I was having but of course... anyways may go ask one of my friends if they could help in this... not that I really want overpowered skeles ;)