>Will I wear out the LED’s quicker if I used power injection on all 20 rows?<
You won't hurt anything. You are just putting shorter and/or fatter wires in parallel with the conductors built-into the power strip.
>Would the ATmega328, capacitors, resistor or crystal be at risk if the LED strip actually did pull 24 amps? Is the 10uf capacitor still acceptable with a current that high?<
That should be OK but I THINK Adafruit recommends a 1000uF capacitor with ws8212's. The power supply supplies the current. And you should have a 0.1uf "bypass" capacitor "close" the ATmega chip. That's pretty standard for chips of any kind.
> If I did accidentally go with a PSU too low (say, 2 amps), could I end up damaging my components by drawing the maximum rated current of the PSU? Or would the LED’s just simply be less bright?<
If you exceed the current rating on the power supply the voltage will drop and "bad" or "undefined" things can happen to the power supply (but not to the connected components). A well designed power supply will usually just shut down but a fuse could blow or something worse could happen. When the voltage drops the microprocessor program might glitch or crash and addressable LEDs also each have a chip that could "glitch" and behave unpredictably.
The dimming is also based on PWM so at 10% brightness the average current is 10% but the peak current is 100% for 10% of the time. Most power supplies will work OK as long as you keep the average below the power supply's maximum current rating but it's not guaranteed.