Have you considered an Aussie for trail riding? I find them very comfortable and secure. You can order one with serge panels which are more like an English saddle on the bottom, and they tend to fit better, IMO. Down Under will adjust a saddle according to your wither tracings, and then there may be options to awl one side so there is less padding. I think they also have a saddle pad that you can shim on one side or the other until you get the horse evened out. I ordered one and am in the process of sending it back bc I need a bigger size for me, and I also sent pics so they can check the fit on my horse. Their shipping is fast. Australian Stock Saddle Co. does the same thing, but I saw fewer saddles from them in my price range.
If you go with the Skito pad it sounds like you would need a bigger saddle to accomodate the shoulders, and then shim the loose side, but someone at Skito would know a lot more.
As far as treeless goes, if this next Aussie of mine still causes problems I am going to try a Bob Marshall Endurance, which I read is better for wider horses. It's pricey and you need a pricey pad, but they really hold their value. They also take 8-10 weeks to get, unless you are lucky enough to score one on Ebay. They are also less pricey than many treeless though.