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I'm wondering more about what ingredients create a large number of discrete super cells well ahead of any QLCS that eventually forms and sweeps through an area.

This has to do with the amount of instability, shear, and lift available. A multitude of these minus a CAP on the atmosphere will allow air parcels to rise uninhibited to create supercells. The cold front itself creates it's own lift, so any remaining instability is used to create the QLCS. A CAP is just that, a cap of warm air above the surfaces that inhibits air from rising freely through the atmosphere.

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On April 27th the convective mode was almost exclusively super cells

While this is true, remember that there were 2 rounds of storms prior to the explosive supercell development. The first, that morning, was an MCS (Mesoscale Convective System). It was basically a small low pressure that formed on an existing QLCS that moved through creating it's own lift and shear. Hence the alarming number of tornadoes that morning. Remember, Cordova, AL has the distinction of being the town that was hit twice that day. An EF-3 with the morning line, an EF-4 with a supercell that evening. When that first line moved through, a second line formed and went through the Tennessee Valley area around 10ish. Then, the ingredients were such that instead of being used up and reloading slowly, it was like an entire new and untouched area of instability just took it's place. The shear from the upper low then was so high that supercells formed and were not allowed to ever line up to to the shear keeping them separate. That's why the storms that day were moving at 60+mph. The meager line at the very end was the front itself, allowing our fears of more storms to subside. The atmosphere was just spent at that point..as it had been worked over multiple times.

The number of supercells that day wasn't extremely rare, but the amount that were producing violent tornadoes was. You can watch videos of those tornadoes and see how violent the atmosphere was that day. Not only were they multiple vortex tornadoes, but in some, you can clearly see horizontal vortexes flowing into them as well.