Very sorry to hear your first experience with your set-up did not go as you had hoped.
I do not shoot the camera you mention so I cannot comment on the specifics as to why your set-up created the problems you experienced. I do however shoot an Aquatica housing and this one would be my fourth.
I have had only one time when water has got into the housing and that was user error and rushing the process. I was cleaning the housing for a soak at home after a trip and did not watch as I sealed the backplate onto the housing. The o-ring pinched. Fortunately no camera was in the housing and I learned from my mistake. User error.
I have hundreds of dives in Bonaire and am well versed with the sand on the o-ring. It is a fact of shore diving and the wash/surf that occurs in the shallows. But, it is impossible for the sand to get in and around your o-ring unless you have not seated the back plate and o-ring properly. I make a habit of continually checking my o-ring before replacing the back plate on the housing every time I open it. I do not necessarily remove the o-ring but check it for anything that may have been grabbed by the grease etc. I have also found sand on the outside edge of the o-ring with my Seatool housing and my Sea and Sea.
I do know that certain levers and, sometimes even the zoom gear can catch on another lever or arm when inserting the camera. (This can happen with any housing) I use the baseplate as a guide as to how well the camera is seated. If the baseplate does not slide easily and the rear locking mechanism fit perfectly without any pressure then I know the camera is seated as it was designed to be. The backplate should sit easily on the back before locking it down, then I know everything is insert properly. I then give it a quick check and fire a few shots and once in the water I always check the housing on first dip for bubbles and or water.
I should also mention I have subjected my housings to some of the most difficult conditions, the arctic, and while sand is not a concern ; ) I have had absolutely no problems with o-ring seating even in the harsh, cold conditions.
As for the extension ring it is unfortunate that the sales person did not properly explain the need for the extension ring. You did mention it was one of the first off the line so it is possible the salesperson was not aware of the need. I do not have the mini dome so I also can not speak from personal experience with that piece of gear.
Sorry to hear about the issues you experienced. I know we all hate to not be able to create images on dives but I am sure lots of us have made errors in housing set-up that have prevented us from shooting on dives. I remember pointing out the Nikon lens cap to a good buddy one dive a hundred feet down.