First: would it be trolling when someone, in responding to a valid on-topic point that was made, instead of addressing it just insults the person who raised it and tells them to "grow up"? - In my opinion, someone telling another member to 'grow up,' isn't trolling. Although, context matters - if the person who posts ''grow up,'' has had a genuine argument with the other member, but the other member keeps feigning ignorance (hypothetically lol) - then, that person might be tired of explaining themselves.
Second: if the person who made the valid point, and who had been insulted, tells that other person to please not troll, and then explains the appropriateness/relevance/validity of the point that the other person didn't address, is that trolling? - Again, just my opinion, but no, this isn't an example of trolling. I feel at this point, moderators should simply issue a warning that tells both to get back on track with the subject, and ''discuss the topic, not the member.''
Third: if the person who had insulted the other member then brushes away the request not to troll, proceeds to insult the person again, while still not actually engaging with the valid point that had been raised, is that trolling? - Still not trolling, just reacting to the other member, and I'd refer to the above, it would be helpful for a mod to insert a simple ''don't name call each other, keep the topic on track.''
Fourth: if the person, now having been insulted twice, explains why he considered the person to have been trolling, again asks the other person not to troll, and again explains why the point he raised is valid and why the other person has not yet addressed it with their comments, is that now to be considered trolling?
If not all, which part in particular is trolling?
The willingness and encouraging to engage on the valid issue that had been raised?
Asking the other person not to troll? - It's still not trolling imo, because if the person telling the other (well intentioned?) member to 'grow up,' and the discussion starts to take a different direction, and everyone else starts piling on the member who is posting things that might be in good faith, then it's really just a distraction and the mod should continue with the warnings. I think if someone joins a science site to post things that they know are not facts on whatever topic it is, but they push it as though they were facts, then they should expect push back. ''Grow up,'' strikes me as a comment someone makes when they're worn down, and have no interest in trying to convince the other member of anything. I think trolling would be if the person being insulted is purposely posting ignorant things, pretending to be genuine, only to get under other members' skin. It's hard to definitively pick out trolling under just one example - for me, trolls reveal themselves when the behavior is repeated, and starts forming a pattern.