enchanting items only when in main hand

i think you would have to set up an 'on player tick update' procedure that scans the players inventory, adding a level of sharpness to the item if held, and then removing sharpness when it is no longer held and adding back the original level after the fact.

if item is enchantable will not work on items that are already enchanted i believe, 

also when you're adding and removing enchantments from an itemstack variable it will only effect the stored variable data, not the actual item that the variable is based on

i think it would be better to do item stack iterator, there's a block that lets you run as item stack iterator for every item in a player's inventory. If the itemstack iterator is enchantable or enchanted and item stack iterator = item held in main hand then you add one level, and give it an nbt true/false tag set to true. then for every itemstack iterator that isn't in your mainhand and has that nbt tag set to true, set it to false and remove 1 level of the enchant