What are the most efficient uses for the first 100 gold that you get

I saw 4 pirate caves in the nexus the other day, so I conclude that yes, it is possible to use a dungeon key in the nexus.

Check the options. There's an option to set the map's default angle to 45 degrees (default) or 0 degrees as well as some options to change hotkeys and there's where you'll find the defaults for rotating the map (which I don't know off the top of my head but I believe they're Q and E). You have to unlock the map for rotation in the options though before you can use Q and E, this is of course assuming that those are the right keys to do it with.