Regarding your first question, the only way we could even determine if it a meaningful question to ask is if we demonstrated conclusively if gravitons exist or not, which would require a quantum theory of gravity that we currently just don't have. Great question though.

Regarding your second question, yes. And as we increase our number of gravity wave followed shortly by EM wave detections, I'm willing to bet that the difference between the two will become a very useful tool in the cosmic distance ladder toolbox.

https://en.m.wikipedia.org/wiki/Cosmic_distance_ladder