oknow said:
In this thought experiment, there is no need to add a gravitational field detector because the question is why the lab and the experimenter standing adjacent (the environment) do not serve as such.
This is not correct. The lab and the experimenter do not exhibit any observable response to the molecule's gravitational field. So they can't count as a molecule gravitational field detector.
oknow said:
if we place a 10 kg lead ball at one corner of the lab, the gravitational field effect of that ball on the lab and experimenter will be different than if we place that lead ball at an opposite corner. That field reveals the location of the ball.
Does it? Has this experiment been done? Can experimenters use lab equipment to find a hidden 10 kg lead ball using only its gravitational field?
More pertinent to this discussion, has an experiment been done in which a lead ball is in a
superposition of being at one end of the lab and the other, to see if the lab and the experimenter will decohere the ball? Of course the answer to
that question is no.
oknow said:
I imagine potentially that the gravitational field of a molecule is too weak to trigger quantum decoherence via the environment.
That is what the experimental results indicate, yes. It is also the point of
@Reggid's response to you in post #2.
oknow said:
If that is the case, what is the largest mass for which that could remain true? Up to Planck mass?
We don't have a theoretical answer to this question because we don't have a theory of quantum gravity.
We don't have an experimental answer to this question because we are unable to run double slit experiments, or anything like them, with quantum systems larger than, IIRC, buckyballs (60 carbon atoms), which still show interference patterns.