I need help with some things

I'm almost sure that for entities to drop items from your mod it's only necessary to change their loot tables, I don't think it's difficult I remember just having to select the type of loot table for entity and put the name of the desired entity

Ex. If you want to change the loot table of a chicken just create a new loot table select entity and put in the name ''chicken'', but be clear that it will only drop what was put there and not what normally drops so if you still want it to drop feathers you need to put that in your new loot table too

The second can be done with procedures and is very simple, just put to identify the inten on the player's hand 

First: you could use a loot table for a vanilla mob like this

https://www.mediafire.com/view/bxr3g189fklo0cb/image%25287%2529.png/file

This should make a chicken have to drop a grass block (although it might completely remove the previous loot table)

Second : Use key binding to make it that when key pressed (or released) then do a procedure that checks for a specific item in your hand and then does something that you want it to do

Like this : https://www.mediafire.com/view/pfqwehud1mrc7wy/image%25288%2529.png/file

And this : https://www.mediafire.com/view/8wsv9lqkaus4fw4/image%25289%2529.png/file

making sure to replace the item (diamond) and what happens (replacing the drop diamond)

another way to do the mob item drop make make a procedure that uses the Entity Dies Trigger, the an If Entity Exists block.