I'm not really clear on the rules of when someone who is called for a function during davening should wear a tallit gadol. I have seen people (including those under 13) put one on when doing gelilah on Shabbat, but I have seen people over 13 not put one on when doing gelilah on a weekday. In some places the chazzan is told to put one on for mincha, but not in other places. Today at mincha I noticed that when people went to the bimah to hold the Torah and recite the Kel Malei Rachamim, they did not put tallitot on (and I don't recall if people put them on when dancing with Torahs on Simchat Torah).
The other questions talk about kavod hatzibbur (which would require counting people and figuring out if a majority is already wearing one) and kavod hatorah (which seems only to apply sometimes), but there seems to be some inconsistency. Does it all boil down to local minhag or are there consistent, overarching rules that explain all cases?