2015 engine, let 12v battery die, first start rough - is this normal?

Yes, that's exactly what happens; various ECUs can lose various saved values, but in particular, the ECM loses its "ISC learning" (idle speed control) value. The "ISC learning" procedure in the manual is not much different from what you did. If you want to be fastidious about it, you first clear any DTCs, put the car in inspection mode, with all accessories turned off, and start the engine and let it warm up without touching the go pedal at all, and it should have stopped running weird by then. But it's not as if it won't eventually learn the right value anyway, even if you aren't following those steps exactly.

If it shut down quickly (because you weren't going anywhere) instead of fully warming up (because you weren't in inspection mode to keep it running), you might want to go back and give it that chance to fully warm up. The smoothing out just before it stopped might not mean the ISC learning was done, but just that it shed the load of battery charging for those few seconds before shutting down.

If you start it up in inspection mode and it is still disturbingly rough while charging, you might shift to N while it's warming up, to take that extra load off of it until the learning is complete.

If you have Techstream or another scan tool that can show the ECM data list, you can look at the "ISC learning" (Compl or Incmpl) to see whether it thinks it has completed the learning yet or not.

 

Whenever I've had the battery disconnected (and no memory saver attached), during the next few start-ups there'll be an unusual revving up, for about 3~5 seconds. Pretty smooth though.