Welcome to UKY, though sorry to hear it is through these circumstances. I hope you will be able to find a way forward with your lawyer.
As KFdancer mentioned, you must be in the US to submit your spouse visa application, and while the application is processing, you will not have your passport and entry to the UK (ex. second passport) while an application is in process voids the application.
If you apply for your spouse visa right now, you won't be able to travel to the UK until it's sorted. You'll almost certainly not be able to make the dress fitting, but *might* be able to make it back by September for your ceremony.
If you hold off on applying to try and visit the UK first, you won't be able to apply until back in the US. You could make your dress fitting *if* admitted as a visitor, but in return, you're allowing less time for your spouse visa application to be processed. It might not make a difference, but something to keep in mind when considering your options.
Regardless, I think a top priority is to start double checking the requirements for the spouse visa and getting your documents together. I know you said you submitted an application for FLR(M) while in the UK, but be sure you definitely have the documents you need. Your supporting documentation might not have been examined thoroughly once the caseworker noticed you were in the UK as a visitor and already knew the application would have to be refused.
Outing myself as a total geek, I have friends who do cosplay and theatre and costuming. When they are doing a costume for someone not local who cannot attend an in-person fitting, and measurements won't cut it, they do a duct tape dummy. Essentially, you get a friend to mummify you in a few layers of duct tape so that it becomes semi-rigid and will preserve the outline of your form, and then cut you out in the back. Tape the back up where you were cut out, fold the dummy, and mail it to the costumer. The costumer can stuff the dummy or prop it up on a stand, and fit the costume around the dummy which will essentially be a shell of your form. I know this is totally weird and left field, but thought I would throw it out there in case it's a potential avenue to explore.
I hope you can sort things out, sending best wishes!