I assume that this is basically just not worth pursuing for small-scale orders (e.g. $15ish for Ciglue), but for larger ones what are the reasonable approaches for scenarios that don't involve stolen card fraud?

Notably disputing a credit card charge is completely independent of whether someone owes the debt, the credit card is simply a convenient way for that payment to be handled. What's the point where other collection methods make sense? As an example, if you're consulting for someone and they pay you $x,xxx via card then charge it back, at least in most of the US I believe it's legal for you to do your own collection efforts and contact them repeatedly (this changes if you sell the debt and it's a third party attempting collections).

Correct, the debt is still valid.

You can try to collect through persistence, or take them to court, get a judgment, and then a court ordered collection. It all depends on the value of your time.

I’ve heard rumors that some merchant agreements with processors may include arbitration clauses for recovering chargebacks, but I’ve never seen it personally.