resistances can be found in
database\records\game\balancingadjustment...
files
I use Grep but maybe you can use GDSearch from [Tool] [Modding] Grim Dawn Modding Suite for searching (not sure if it works now but some other features definitely do). Here I wrote how I set up Mod Checker, Anything special needed to use a .pfx as a projectile? - #14 by tqFan which is another feature
How about changing the Familiar skill instead of giving a modifier to some item? that should be simpler
This I don’t know because I haven’t done it yet
PS You can also check out the modding section
on Discord, it’s pretty active Discord