I think you have a 2 fold problem here which will seriously hamper your efforts. First is your weight. Even with dropping weight you are at or over the designed weight limit of most if not all bikes. Second is your riding style. Not being able to get out of the saddle means you are stressing any frame (specifically the rear) in ways that it wasn't really designed to be stressed. Bearing that in mind I think you should be touching base with as many bike companies as possible and seeing what becomes of it. Contact them with your issues and see who gives enough of a damn about your situation to at least get back to you, and even provide you with some guidance or, better still, a solution to your problem. Talk to full custom builders too.
Outside of that, don't stress about the material. A material alone doesn't make the frame, the quality and way it is put together determines the strength and how it performs. You could easily make an alloy frame that is stronger than Ti or steel by using the right tube profiles and thicknesses, so keep an open mind WRT frame material.