Could the installer have hooked up the Alpine output from the speaker level outs? No, that's impossible, right?

Unlikely if he's any good, but way to many "professionals" just aren't... So I wouldn't totally rule it out.

Could I get a de-amplifier for the Alpine so I could get more gain on the amp?

Sure, look for RCA attenuators or build your own (they're nothing but simple voltage dividers)

I thought I read reasons for max gain settings on amps. Any links so I can avoid hurting my amp?

Too high a gain setting (same as too low a sensitivity setting, which is what the gain knob normally is graded with... for instance 8 to 0.25V) will make the output signal clip (bump the output signals "head" into the roof / the max voltage it can do; and flatten the normally rounded tops and bottoms of the sinus shaped signal, making it look more like a square wave) Clipping can lead to the amp putting out twice the RMS rated power (a square wave with amplitude X volts has got twice the power of a sinusoid signal of the same amplitude) which the spake rmight not be rated for. It can also generate a fair amount of harmonics (a perfect square wave is made up of an (infinite) series of sinus waves of decreasing amplitude as the frequency goes up; so your tweeters might also be receiving more power than the system normally feeds them...

Or am I just using my empeg incorrectly as always? Am I SUPPOSED to only have audible sound from -20dB to 0dB, leaving the myrad of volume increments below -20dB to be too quiet to hear? (no, it's not broken, just too quiet).

Depends - if the 0dB setting is as loud as you ever care for, well then you're golden... But to me -20dB is a bit early to have no audible response...

/Michael