"I do a MyoMalt similar to Hammer Perpetuem. There's a topside to how fast the body can take in malto and burn fat with the balance made up by depleting glycogen. For very long exertions, you shouldn't burn faster than the glucose + fat rates, or at least pace it so you rest just in time for avoiding the bonk."
There is definitely a limit, and the quick way to keep your burn rate in sync with your intake of malto sufficient to support the metabolism of body fat is to adjust your pace. Of course, the way to be able to sustain a faster pace is to increase your VO2 max, which you have clearly figured out. Fascinating project, isn't it? Each body is an experiment of one. I'd say you're on the right track and will get it figured out fairly soon.
"I've played with electrolyte balance as well and found that limit - suddenly thirsting much more and getting stomach upset - too much salt."
Have you ever experimented with Morton's Lite Salt? It has ~280 mg of Na and 350 mg of K per 1/4 tsp. I've had very good results with it while training here in the Cascades and also at altitude down in the Sierra. I use 1/4 tsp/liter, but that is just me. I take my Ca and Mg at either end of the day, but that is just me. According to what I have read, they are not depleted nearly as fast as Na and K during exercise. YMMV, of course.
"Mind you, this is after 30 miles of 3.5 mph without pause up and down the Marin Headlands/Mt. Tam."
Now that is impressive! I did a lot of running in that area back in the early 80's,
most of it in the 13-16 mile range at a pretty good pace, but also a fair number of 20 mile plus runs. It is very demanding terrain and, based on my personal experience, I can say you are functioning at a very high level of performance. Tinkering around with your pace and working on your VO2 max should eventually get you where you want to be, IMO, but for the distances you are doing, you will be forced to rely on a combo of glycogen and dietary carbs. No avoiding that, but you should be able to rely on stored body fat for the fat component, at least for a few days in a row. Extended hikes are another story.