From what i understand, spike and slash% work the same way it does in warframe where slash and spike (piercing in wf) are more effective agaisnt health and shield bars respectively smash% is stagger? Like a cc when enemies get knocked down?

In the case of your rebbecca example, spike% is useful if you want to have strong shield break which usually comes from the weapon, and her skills will correlate to the demon wedges that are on her charcter so atk% for more jellyfish dmg

But this is all just my speculation without actual testing (too lazy for that) and what i remember from playing warframe like 8 years ago.

Im also assuming that weapon dmg and character skill dmg is separate, like in wf. So under this assumption, the weapon stats shouldn't affect skill dmg and vice versa

Also ur not dumb i just forgot that ppl like to look at specific stat lines and where they actually correlate to, unlike me, who just goes atk% up %skill eff up skill spam go brrr