"So it seems the current draw isn't an issue."
Not with an LCD that has no backlight. A backlight can pull 40-120mA all by itself, way way more than the LCD does.
"I could set an alarm ...Seems even more fool proof than aligning the analog needles."
Depends on the quality of the fool. (G) An analog needle relies on someone looking at it. Digital alarms involve whether you can hear the alarm over engine and background noises, and multiple stages of electronics and wiring that can all fail. Foolproof? I'd suggest the digital and the alarms require less attention, but they've got more ways to fail. Or, more ways to get your attention.
"Not clear on the "floating negative output" they're looking for an alternator which is NOT grounded to the engine block, but has a separate negative lead. Don't know what you've got. AFAIK
there should be no reason why they need to install the current shunt in the negative versus positive line, since a shunt works the same way in either location, and the polarity of the voltage drop across the shunt doesn't matter. I would guess that because of the way they have set up their alarm circuits, they've painted themselves into a corner requiring the shunt to be installed in the negative lead. It shouldn't (dirty word) be that hard for them to design a circuit that can be used in either side, positive or negative. You see the avantage of fancy digital electronics now, huh? (G)