Dunno, but #3 seems to be the most plausible of the group. No t saying I believe it. Just that IF the explanation was one of the four then it is the strongest candidate.
One comment though about interstellar travel: if we are talking about going faster than light or “bending space/time” then it is impossible to conceive of how it could be accomplished.
Slower interstellar travel is however almost with our grasp even now. At a speed of 1/10th the speed of light (admittedly very VERY fast)a trip to a star 63 light years away could be made in about 630 years.
All you need is a self contained planetisimal in which generation after generation of spacefarers will be born and die until the craft reaches any promising stars, where it parks in orbit (maybe like that of 3753 Cruithne ) and sends out smaller craft for exploration.
Interstellar flight is pretty hard, but yeah, who know how many asteroids there could be in our solar system that are from other solar systems BY DESIGN!!!
It could be the drive to colonize a new world dies off as soon as a race gets accustomed to living in a sub light hollowed out asteroid....???
So instead of colonizing they would just want to visit and explore and go back to the their hollowed out home atseroid and they would possibly spread to other asteroids.
If they can manipulate gravity they can manipulate spacetime and if they can do that then FTL insterstellar flight is a likely possibility.
It’s all quite fascinating...