Ask Mr. Smarty Plants

While widely known as Monterrey oak, Quercus polymorpha should probably be referred to as Mexican white oak or netleaf white oak. Apparently, the term "Monterrey" when associated with this species is a trade name.  In seeking to answer your questions about your tree, we have tapped several resources, beginning with the Conditions Comments from our Native Plant Database on Quercus polymorpha (netleaf white oak)

"Water Use: Medium
Light Requirement: Sun , Part Shade
Soil Moisture: Moist
Conditions Comments: This species is widespread in Mexico and found in a few west Texas canyons. It is a relatively fast growing oak, and practically evergreen in Austin. It is more resistant to oak wilt and other diseases and pests than other oaks. It is also tolerant of drought and alkaline soils."

You may have been overdoing your efforts to save your tree. Native trees should not need to be fertilized, as they are already accustomed to the environment. Too much fertilizer, in fact, can cause damage to the roots. The plant being rootbound could definitely be a contributing factor. You should always inspect a tree with the pot pulled off before you take it from the nursery.  Planting it in September, in the middle of the hottest, driest two years on record in Texas, was also a possible mistake. We recommend planting woody plants in Texas in December or thereabouts. Not only could transplanting in the heat have caused transplant shock, but oak leaves are turning brown everywhere due to the drought. 

Next, we want to determine if where you are growing your tree could have anything to do with its problems. When we went to this  USDA Plant Profile of Monterrey Oak, we learned that it is only native to one county in the Big Bend area.  From Learn2grow.com we got some more information, learning that what you find in the nursery is sometimes a hybrid or mix of several other oaks, perhaps less disease resistant. From the Madrone Nursery , we excerpted this warning: 

"Note: This is one of the most commonly mislabeled trees offered for sale in the nursery trade. Often it is a hybrid form with Bur, Chinquapin, or Red Oak parentage, making the trees more susceptible to insects and disease."

Remember, this is from a commercial nursery, but other sources indicate that it is a valid warning. 

And, of course, there is always the threat of Oak Wilt which hangs over oaks in Texas. See this website from the Texas Oak Wilt Information Partnership, in which the Lady Bird Johnson Wildflower Center is a partner, for symptoms of Oak Wilt and what can be done. 

We have tried to find ways for you to understand what is going on with your tree, but we are not plant pathologists and diagnosing or prescribing correction for a plant problem from a distance is nearly impossible. The people that are trained in this sort of thing can be contacted at the Texas A&M AgriLIFE Extension Office for Bell County.  They will likely recommend you have a trained and licensed arborist look at the tree, as do we.