I can see your point. Sure, they could have just taken it from him when he refused to give it to them after the two trees died, but it wasn't theirs, so they didn't. Plus all those other reasons

Good to see the valar have some morals. Still, I agree that at this point the trees could not be saved. However, the simarils would see be of some use to the Valar. If not to obtain their light, then just as some sort of memorial. This kinda brings my B options. I highly doubt they would be unaware though, for an object that had ruled the fate of the first age.