.......For even stronger gluten development, King Arthur also offers a bread flour with 12.7% protein.
I would tend to agree as off the grocery store shelf bread flours are targeting less advanced preparations including bread machine usage and the protein content and other formulation does not usually produce the result you're looking for.
King Arthur flour is well renowned for its quality for advanced home baking; and if you have someone willing to share some commercial bread flour with you, consider taking advantage of the offer.
Other sources of good quality bread flour could be sometimes in the warehouse stores, although not reliably consistent supply. If you happen to be near one of the few Costco Business Centers, you would be able to get commercial bread flour there. I've occasionally seen King Arthur flour sold in WalMart, but to be sure of getting it fresh, order from King Arthur directly.
To address your problem of developing an open crumb, I would start with a bread flour know to have a protein content of 12.3 – 13.8%, and follow the recommendations in the King Arthur article referred to earlier.
Consider calling the King Arthur help line for additional input, as they have access to some knowledgeable resources.
Will look forward to hearing of your progress, as high elevation baking is an interesting topic!