Have you ever thought about it that you might have a design flaw yourself instead of your horn?
Your problem sounds familiar to me.
Apparently my hands seem quite wide, which makes the traditional gripping position of the left hand too cramped. Besides that, the outerslide sliced skin off my hand every time I pulled it to 1st position. I have played both Edwards and Getzen horns many times and there seems to be nothing wrong with them but for me they have the same gripping problems which I had with my Bach. I have that problem with many brands. So, it's my problem. Not that of the horn. (The only horn that I had the opportunity to play, that did not have this problem was the Thein Ben v. Dijk model. But that one causes some serious pain to your wallet.)
I HAD these problems because I was able to solve them easy and cheap. I started out with relocating the pivotpoint of the second valvelever. I then shortened the lever so that I could use it in combination with the "Douglas Yeo-grip". I already used a backhand support and that whole combination worked quite good immediatly.
But, I still wasn't good enough. I found out that there was still some room between my gripping hand and the slide itself. So after a three hour practice my hand and arm were painfull again because I had to squeeze too much to hold the horn. I saw photographs of Mr Yeo and to me it seems that I am bit taller than him. So chances are that I have larger hands than him also. Which can explain why his grip works for him and not for me after some more alterations.
I solved that problem by wrapping handlebar tape from a roadracing bike around it. I use it to fill up the gap between my hand and the slide and it works great! The corktape quality that I use follows the shape of my grip and stays in that shape when I don't hold the horn. It is a cheap and cheerfull material that doesn't do any damage to your horn. I tried the fancy leather option but it is too thin for me and my slide turned green underneath it in a matter of a few weeks. I have to admit. It doesn't look the bizz, but my whole horn looks crappy anyway.
The whole opparation sat me back around 90 to 100 Euro's and gone were the problems. I don't know how much that is in Dollars but you can figure that out yourself, I think.
I cannot say that this it solution for you too. But is relativly cheap to do and if you have the pivotpoint of your second valvelever already underneath the slide receiver instead of sideways, it saves you some more money. A backhand support is almost always a good investment although the Rath support is ridiculous expensive. I use one made by my technician himself. As for the Douglas Yeo-grip: If you try it with some tape it might work out fine but that is up to you to find out.
Good luck solving this problem.