If you have to put the diagonal right up against the end of the focuser and have no more adjustment it would seem that you need to shorten the tube to come to focus in the middle of the focus range. The light path of the diagonal will add maybe 100mm to the optical path length. The camera should add only 55mm or so to the focal path so I think it should reach prime focus as is. That allows for a 20mm Nagler to be used. The larger end fits nicely in to the focusser on the scope.
Prominences are easy, surface details are another matter.