OK... time to answer my own questions!
retrofuturist wrote:1. To what extent do you, or have you, experienced weltschmerz in the past?
Plenty. I have a (thankfully diminishing) tendency to be critical of the way things are, and how far they are from being optimal, and how they are corrupted by the ignorance, greed and/or delusion of others. The closest I've ever come to having a mental breakdown was because of weltschmerz in the workplace. This is also why I used to be a committed socialist. I still have lefty leanings, I'm just not so attached to them now such that they would cause me suffering, if not realised. I've also experienced plenty of weltschmerz with respect to music-related things too... mainly relating to the popularity of rubbish music, and the lack of recognition for quality music. The potential objects for weltschmerz, as with craving, are limitless.
Even my username, retrofuturist, reflects weltschmerz to some degree... the wish for things to be now (in the future), as per the advanced scientific utopia predicted in the early/mid 20th century. You know, flying bubble cars, space travel, travelators instead of sidewalks, electronic music etc.
retrofuturist wrote:2. To what extent has your Dhamma practice alleviated weltschmerz?
Weltschmerz is caused by craving, and realising this I've been able to let some things go, for my benefit. Things which caused weltschmerz have been useful Dhamma experiences for me, which made my understanding of the Four Noble Truths a very immediate and potent "a-ha" moment
retrofuturist wrote:3. To what extent has your Dhamma practice contributed towards weltschmerz?
Buddhism can create new unfulfilled ideals for us, as Genkaku pointed about, and if we lament over the reality of not yet meeting those ideals, we are bound to suffer. It is a delicate balance working out when one should strive and commit effort to something, versus letting it go. Mindfulness in that process is invaluable.