I love your ideas, I really do, but I think people will abuse the 'Late Join Quest'. Some people will purposely join late to the quest room like 5-10 secs before the boss dies to get a free EXP. But of course people cannot predict from outside the room if the quest has just began or its about to finish.
But in my opinion, we should give it a shot.
If it works good and people will love it = Great, keep it.
If most of the people won't like it = remove it.
Simple as that ^
