This is interesting....
I'd be interested in knowing how much the overall rig length would be reduced. I like the aluminum trailers, but hate the amount of length required to bring the Ibeams together to form the toungue. I have seen some ibeams use a galvanized tongue to eliminate the ibeams having to bend over a long distance. My ibeams come all the way forward to form the tounge and I bet I have 6' from the bumper of the truck to front of the boat. And that's even having moved the winch stand to the all forward position.
Without doubt the 5thwheel hookup will give you a much better turning radius.
I had spoken with Owens and Sons about 5th wheel aluminum Ibeams... They do make them. I don't remember, but it might have been an extra grand over the conventional ball hitch... don't quote me on that.
You might consider the selling of your trailer and buying new vs getting a possible expensive custom job done... unless you can do the work yourself... that's a no brainer.
One question also is if the wheel position is the same on a 5thwheel vs regular ball hitch setup.
Would storage length be the same also or longer??
Sorry I couldn't answer any of your questions, but you raise an interesting question.