Spider mites lurve them some buddleia, particularly if it's good and drought stressed. I've been able to control SMs on buddleia by (1) making sure the plant itself gets appropriate irrigation and (2) hosing it off daily with a pretty good spray of water, being careful to hit the underside of the leaves, for a week or 10 days. That seems to interrupt the lifecycle of the SMs and they'll remain at a managable dull roar if you make sure the plant gets enough water.

When any plants leaves brown at the edges that is an indication that the plant is transpiring moisture faster than the roots can supply it to the leaves. That does not always mean you need to water more, or more often, but for some reason the roots just cannot move enough water up the plant fast enough. Maybe you need to proivde this plant with some shade since I know that with air temperatures over the mid 70's I require a lot more water, especially when working, than I do with them in the mid 60's.

For the buddleias to get started, they need lots of moisture. I mulch mine with pine bark. I am wondering if it might not have done better in your environment if it was planted in the late winter-early spring. It sounds like the leaves are cooking. Of the mulches you listed, I'd use lots and lots of shredded leaves. Your plant can lose all its leaves and still be alive. To water, make a trench about a foot away from the plant all the way around the plant. Water in this trench and let drain. Keep mulch away from trunk, but use it liberally around the plant out to about 18 inches or more, including the trench. I would put about 4 inches of mulch around the plant. Good luck. I don't know if buddleia is suited to temps of 125 degrees.

More than likely that is from the larva of one of many insects, or maybe even the adults. Are there wee green colored thingys on the underside of the leaves? Do you see any insects on the plant at all?