I've never seen any of the things you mention in my time on the JP server, and some of the points you make do not click or make any sense in the grand scheme of things as well..
4th Recommended (or Featured as it is called in NA) is extremely easy to do as long as you do not do it on the highest difficulty. Looking at your level, I assume you would NOT be doing that difficulty. I hope not, anyway as the other difficulties give an extremely high amount of EXP and can be cleared faster with weaker gear. I've never heard nor seen anyone leave because some people appear to be "new" or are "undergeared". It seems weird, even, considering people are there to level so they can use the said powerful gear.
Why would people leave others in the dust because they don't have Successor (Scion in NA) classes? For what reason, exactly..? What do you mean by that anyway? Are they just killing everything faster than you can, or leaving the Dungeon? Scion classes are more powerful early on than other classes but level out later around the time other classes are getting stronger so It's to be expected they kill faster. The leaving part makes no sense, as people don't care what class you use. They really don't.
The final part is the weirdest; Why do they purposefully ignore the enemies on the platform and make SURE their attacks don't hit them? Why? What is the point? I can see ignoring the adds just to do DPS to the Castles arms to get the run done faster to start another, which is the whole point in farming 4th Featured for EXP. ((For people who don't know, there isn't that many enemies on top of the platform and they also give barely any exp compared to the rest of the Dungeon so actually killing them could be considered a waste of time))
A side note for people who never played JP: 4th Featured are an amazing way to get EXP usually. Some Dungeons aren't worth farming but most are, especially the Castle he mentions in his post. The Boss itself gives tons of EXP. You don't even need Advanced Quests anymore once we get the 4th Featured.