For one, an enderman farm should be at least 200 blocks from the main island. And you say 'we', which makes me assume you are on a server. Its probably the server being overloaded in some way or too many mobs in other places.
Ya this has happened to me several times, its annoying but I'm sure Mojang will fix it.
Could you provide a snapshot and some redstone linked to the button to prove its not working? It could just be a visual glitch, and if this was on a server it may have been actually there but not rendering correctly for you.
This is due to either skins being down or your MC didn't download skins right upon startup. This happens all the time and is not really a bug. If you want to see if skins is down go to https://help.mojang.com. If skins are not down try quitting MC, closing your launcher, and re-opening it.
I believe this bug has been present since powered rails were added. I've tested it back to 1.0 but not in beta 1.5 because of no creative mode back then. I'm surprised such an old bug hasn't been fixed lol.
This is not a bug, please don't post things like this here in the future. Try using MCedit and deleting the command block minecart.