What is the fine line between quality and quantity and how does it affect merit?
It does not affect merits in any way, because you can see a quality content with few lines and even less than a single one and gets more merits, while a whole page content may not receive a single merit, this shows how important a post quality is base on the forum standard, while giving a merit is base on the readers discretion in deciding for which post deserves being merited or not.
Is there a chance that some people are just posting to abuse the merit system without having anything useful to say?
If you know how hard it is to earn a single merit, then you will also know how important it is for you to dish out merit on any post merit worthy or a less quality post that does not deserve being merited.
Should people have an alternative way of being rewarded with merits besides posting?
How could this look like, and how would you design the system so it can't be abused?
Merit is not moderated as you think in such manner, if you don't post, then how could you be merited?
Abuse seems to be inevitable, but once you sight one that looks like merit abuse, then report such, once you don't sell or trade for merit or merit yourself, you're still on a better edge.