Hello,
1) I think SDK contributor should be able to just checkout some tag with everything related easily, otherwise contributor should manage a lot of projects, use workarounds like cbuild scripts.
2) we are tagging everything even if it's not changed because we have some automation on that it allows us to do frequent releases, I agree that it's overhead but it's a big price for being able have release automation. Btw it can be a nightmare if number of plugins will be more than 100, for now it's ~25. Also if we will have everything as single project we will don't need anymore maintain our release automation, and it will allow us to have a job on CI for release, which can be scheduled or launched by any allowed person, which is very important if release team will not be able to work.
3) In fact our devs spends a lot of time on that, for example new runner panel was developed in more than 15 projects in the same time, and it's not that easy to do development this way. Also it is not a trivial task to merge all those projects in master, I think it can be much easier with single project approach.