Tom,
If you don't find a trade, here's some ideas that might help.
1) You could replace the SS BP with an AL one, which should resolve the weight issue.
2) Comfort should improve by having someone familiar with BP/ harness fit it to you. Plus, it will be comfortable once in the water. You can also add neoprene shoulder strap pads to the webbing to improve comfort when out of the water.
3) I had a rotator cuff problem myself so I know how that limits range of motion. After you get the harness properly adjusted to you, you might consider adding a quick release buckle to one shoulder, like many conventional BCs have. The webbing will no longer be continuous, but so what if it allows you to use the rig in comfort.
When I first got my Halcyon MC, I too was a bit concerned about the comfort when trying it on land. But after getting it fit to me and actually diving it, it's very comfortable and my concerns have disappeared.
I'm sure you will find a solution that works for you, or put the rig on eBay and you'll get a very good price, look at recent completed sales.
Don