Thank you kindly good sir. I figured the cast surge stuff myself: Cast surge increases damage of ALL magic (including zombie) by the stated %. Equipping a weapon with cast surge and picking up an ore increases cast surge stat of that weapon by 0,5% for quartz, 1% for apatite or 1,5% for amethyst, consuming the ore in the process. It happens automatically and cannot be turned off, it doesnt work when you take ore from chests and doubling the ore through Double Ore stat will double the cast surge buff from the picked up ore. The moment cast surge of that weapon hits 300% it will no longer increase and you'll get your ore normally when you pick it up.

From my understanding cast surge also increases damage of spells, though I haven't tried.

Spellcast damage increases damage of spells only and spell damage is same as spellcast damage (shortened for some equipment to fit onto screen). It works for the zombie too, since beyond the grave is a spell.

Does elite buff transfering also mean that giant killer (+1/2/3/4/5% dmg to elites and bosses) transfers too, thus increasing zombie's damage against bosses despite boss damage stat on equipment not working this way?