Sorry for chiming in late, just getting back into things here (was out in Indonesia for a few weeks without internet on the boat)
The locking collar is removable, but has Diverdoug1 says, it can be annoying to switch all the time, the macro port does not require one, so there would be the extension for the 16-35mm and the 8-15mm to consider.
The 18459 is a really new extension, just being introduced so it is not common knowledge yet, but it is available.
The +2 can and will help with close focusing on the projected virtual image of any large dome (8" or bigger).
The 9.25" Megadome has a built in dome shade that will show up at the widest range of the lens when used with a full frame camera, but it is fine at 15mm