Help Stopping Heel Slippage in Trail Shoe

You might try lacing the shoes in different ways. For examples see:
http://www.inov-8.com/lacing.html#lacing

You might try using a thicker sock on one or both feet. Heel insoles can help on boots but may not be workable on trail shoes. Hope this helps.

I wonder if you could build up a few layers of moleskin or something similar on the inside of the heel. A sheet, snipped in the right places, should fold into a cup rather nicely.

I'll look in a book I have used for years for stuff like this: "The Runners Repair Manual". It's full of things like this. I'll let you know if I find anything.

That idea might work.

I tried a green Superfeet insert just to see if anything along that line would work (no, I'm not actually going to ever use them in that shoe) and it didn't. I'm going to explore the possibility of buying two different sized shoes. Highly unlikely that the company or a retailer is likely to do that, but you never know. I really liked those shoes.

One thing to keep in mind is that footwear sizes are not 100% accurate. Case in point; last year I got a new pair of snowboarding boots, which by the nature of the sport requires a perfect fit. The store had about eight pairs on my target size, so I proceeded to mix and match from all eight pairs until I hit the perfect left-right combo. They were all a size 10, but some where more than half a size apart.

Next time get every shoe they have on a particular size. If they have twenty boxes, you want to see them all. Find the largest for your left, the right one for your right, and you should end up with a pair of shoes that feel like they were custom made for you.