2) Are judges allowed to tell people what "they should've done" between matches?
Judges are like people. They exist between rounds and offer advice/analysis like anybody else. Any such advice/analysis should be appreciated/ignored like any other advice/analysis (I know. I maindeck Conjurer's Bauble as a Kill card in Salvagers, and I some times let Ancestral Recall resolve willingly)
Take it or leave it. They don't neccessarily know the ins and more importantly, outs of your deck off hand.
Comments like "you are the Suxxor: you suck" may lead you to end career of said judge, or file a complaint or whatever.
In my experience, a Judge may question or analyze my play from a finished round, and it is "I" who claims that "I am the Suxxor", but that's me: I don't always make the optimal plays. (I am human, and not a Terminator Magic Player.)
whats the big deal? Judges are Magic Junkies too, as far as I know.
This is assumming (as per your stipulation), that this is all done BETWEEN rounds, and not even between games (2/3 is a round)