I would open the mod folder (instead of using GUTS) to explore the files that were changed in the Death Ritual mod you used.  There's likely something that needs to be changed back to vanilla for your Death Ritual skill to work correctly.  I'd check the Skill Tree Override, the Skills, and the Stats in that order.

Also, the way Death Ritual works is that it sets a variable (a stat) that augments the effects of the other three skills.  The other three skills all have either Effects or Stat Watchers that are dependent on that variable.  If any of those skills or their connected Effects were changed, that would prevent Death Ritual from working properly.

From what I remember, I don't think it would change the tooltip.  Most summons' damage tooltips don't change even with level, it just says "+x% damage bonus".

I think you'll need to look at the combat log to see Death Ritual's effect on minion damage.

Non related question.

What could cause a skill to be bugged and doesn't do any damage, at all? The animation still plays, but no damage dealt. However debuffs like stun or slow movement speed still affects monster.

Nvm I figured out. There's some conflicting skilltree.layout   

More question. What does the stats folder for? I've seen alot class mod having identical files in stats folder like characterdexterity.dat for example, but with different GUIDs.

Is it safe if I just choose 1 identical .dat from any class mod and hope that it works for other classes?

  Now that's a tricky one.

I'll just do it and hope for the best, lol. Thanks for answering my questions.