There is another possibility: That we are not seeing the same SJ and derelict at all, but something from the same race.
However, fossilisation never made sense, ossification perhaps. But Dallas wasn't the science officer so his interpretation need not carry any weight.
I'm personally not that keen on time travel and wormholes as that still seems a little far fetched for the Alien universe, but being a fan of the original film only, I'm confident that whatever story Ridley gives us it will be the best expansion of A L I E N in 32 years. I'm hoping for a mature story, one that puts to shame the downward slope into juvenile shlock of the 'franchise.' As long as it does that I can give it a lot of leeway in the way things like long assumed ideas about the SJ are handled.