It's a process of elimination. Try the wider saddle on a test ride. The shop should accommodate you on that score.

But I suspect you have other issues with tilt and position of the saddle rails on the seat post mount. The clue for me is that you say when you reposition on the saddle to sit on your sit bones, you feel more comfortable. It sounds to me that you are pedalling too far forward, with your tailbone on the nose of the saddle.And if your saddle nose points downward, you are more inclined to be riding in that position.

Does your saddle tilt downward? Do you feel inordinate pressure on your hands as you ride? If so, the saddle should be put into a level position. I always advise people that they should be able to sit upright on the saddle while pedalling with next to no pressure on the handlebars (or ride hands free) and not feel that they are sliding forwards on the saddle.

Also, does your bike feel too long for you so that you feel that you have to sit forward on the saddle to reach the handlebars? If so, that's a more fundamental problem of bike size.