Okay, I understand the first point now, and I managed to fix it!

I messed up a bit with the first mod; I mistakenly thought the Normal.xdb files in the Cast folders (\UI\HeroScreen2\Inventory\Cast and \UI\HeroScreen3\Inventory\Cast) were the ones that specify the texture parameters, but they are actually named (BackgroundSimpleTexture).xdb. You need to replace them with (BackgroundSimpleScallingTexture).xdb, but keep the file names the same - Normal.xdb.

P.S. I've attached the corrected mod with the cast icon in the inventory.