There is a new proposal in area 51 about voting systems.
Are questions on voting systems and voting theory welcome here? Under what conditions?
We have a tag for voting-systems. I would find your six questions to be on-topic. Note that some of them might be considered under-researched. Also
"What single-mark voting rules are there other than Plurality?" is likely too broad. List questions are almost always off-topic on any Stack Exchange.
"Why is plurality so popular?" might be closed as opinion-based -- you might have to work to phrase it in an objectively measurable way.
While it may be possible to come up with a voting system that is not on-topic for Politics.SE, it seems difficult.