Git itself isn't decentralized is about people copying it and sometimes mirroring it.
Not sure what you mean. My understanding is that git itself is decentralized insofar as each clone can develop its own history without ever needing to push to the origin, but that what OP is referring to is actually the “distributed” nature of git, where i.e. it’s easy to copy the entire history of an instance.
RCV doesn’t “solve” the issue though. The fact that third party candidates can sway elections to the least preferred candidate is known as the “Spoiler effect”, and RCV is also subject to it.
RCV seems to be objectively better than plurality (what we use now), but it and any other ranking-based voting system are still subject to spoilers. One thing that can actually “solve” the issue though is rating-based systems, like Approval Voting, Score Voting, or STAR voting.
Good video on the subject