Great set of questions. Here goes so hold on:
1) You slow down because you are exceeding your aerobic capacity during much if not all of these fast runs. If the slowing occurs in the 1-1.5hr range then you are slowing because of glycogen depletion and not so much from the accumulation of lactate or increased acidosis. Your 6:30 mile pace is probably right at your endurance limit, often called your lactate threshold (LT) (used to be commonly called anaerobic threshold). This LT pace requires the maximum sustained output of both of your metabolic pathways: Aerobic (using fat or glycogen for fuel) and the Anaerobic or Glycolytic (which can only use glycogen/sugar). At this pace both systems are cranking at their max sustainable output so you are burning through your glycogen reserves fast. When those reserves run low you are forced to reduce your pace. Your brain does this automatically and against your wishes. It is trying to preserve the organism from death or damage so it needs to husband those precious glycogen reserves (the brain can only on glycogen so you can imagine what would happen if completely drained that fuel tank).
Because the glycolytic metabolic system is operating at near max capacity you can be sure that lactate production is high. But at that pace/effort the lactate production is just equalled by the lactate removal.
2) The marathon is an event that is competed at every individual’s aerobic threshold. The aerobic threshold represents the metabolic limit of energy production where fat dominates as the fuel. The world class marathoner has an aerobic threshold pace of about 4:45/mile. The weekend runner also competes at his aerobic threshold but it happens to be at a 10:00/mile pace. But from a metabolic standpoint they are both at the same limit. The world class runner just can crank out so much more ATP/second with his fat metabolism that he can run twice as fast.
As you have discovered in your fast runs you begin to slow at about 60 minutes. Events that take about an hour, like a world class half marathon or a recreational 10km all are competed at that individual’s Lactate Threshold.
As to your last question: Both the aerobic and anaerobic metabolic pathways are each contributing to your overall energy demands at all times. How much is coming from fat and how much is coming from carbs (glycogen) is based largely on your dietary choices and on your training history. I coach top endurance athletes that are barely sipping from their glycogen fuel tank all the way up to 80+% of their max HR. I also deal with many folks who come from high intensity backgrounds how are using 80% carbs at just about resting intensity, like a walking pace. These folks have severe Aerobic Deficiency Syndrome. (ADS).
If lactate production does exceed lactate removal, as happens in short, very high intensity efforts sustainable for only a few minutes, then yes, the anaerobic/glycolytic system will slow and finally shut down as lactate reaches very high levels. When that happens you will be forced to slow WAY down until the lactate clears and the acidity is restored to normal levels. During this slowing process you will mainly be relying on your aerobic metabolism to power your forward progress.
More info in Training for the New Alpinism Pages 101-113.
And this article here: https://uphillathlete.com/what-is-it-that-enables-endurance/
I hope this helps,