Is Dumbledore all that knowledgeable about Horcruxes? Didn't he have Harry go get information from Slughorn to figure out how many he made?

 

That is information about the number of Horcruxes, not the Horcruxes themselves. If Tom Riddle asked Slughorn about them, then there might have been a clue that would lead Dumbledore to discover how many Voldemort created.

 

Being knowledgeable about Horcruxes (which Dumbledore certainly is) doesn't allow you to magically deduce the number of Horcruxes someone has, especially when all conventional wisdom regarding them states that splitting your soul once is de rigueur, and Voldemort attempted seven.

That Dumbledore was able to deduce the nature of the vessels based on his sheer understanding of Voldemort's psyche is a hugely impressive feat.

 

Thinking about it, would you really, really, want to know any more about them other than where and how many, and how to destroy them? They're abominations that shouldn't exist

 

Yeah that's what I thought as well. Dumbledore's morality probably restricted him from truly learning about the darkest of magics while Voldemort has no limits or morality and in the whole more knowledgeable about it.

 

This has me thinking, why do we instinctively measure a wizard's competency by their capacity for dueling and violence?

 

Because that is the most direct way of comparing two wizard's magical ability in perspective to each other

 

I mean I agree it’s the most direct, but not sure it’s the best.
Is Molly really a better witch than Bellatrix?

I’d argue duels are actually a really poor indicator of a wizards competency. How often do most wizards ever actually need to duel?

 

Dueling is one field where you can be competent, but trying to give a universal "competency score" to people seems pretty useless to me. Competency is all about the context.