Regarding fixer and weather or not it is heavier than water. I don't think fixer is heavier than water but fixer laden with unexposed silver salts as it would be when it is being washed out of an emulsion probably is. And I think that is what Picker meant.
And as for poor shadow separation the problem is in the development. Or tather in the agitation. Modern films do not need much agitation. I've swiched from the "ususl" twice a minute to once every two minutes. This equals less development so the time needs to be extended to reach normal scale. This gives the shadow areas time to develope fully.While holding back the highlights due to the standing effect The differences are amazing. And as Picker would say, "TRY IT"
I agree, this is one of the most intersting topics in a while. I wish you all good light and lots of film..