Sounds like you'd want a visible indicator of some sort for each detector if nothing else to be able to tell which one has gone out of alignment. Otherwise, yeah, when the alarm goes off the forklift operator knows to stop lifting and try again. So you'd want to either wire all of the detectors back to a single panel full of indicators, or mount an indicator at each detector. I'd think the latter would be best, because it simplifies wiring to a simple daisy chain, gives you a visual indication that you can see from the emitters as you're aligning them, and if you've got multiple forklifts operating at the same time, when the alarm goes off each driver can simply look down the row to see if they're the one who set it off.
On the other hand, having a central annunciator panel would allow you to add a switch to disable each detector. If the system is really as prone to going out of alignment as you suggest that could be a big plus, so you can mask off a misaligned or malfunctioning detector until it can be serviced while leaving the rest of the system operational. Of course you could do both a central panel and an indicator at each detector, if you want to maximize the cost and amount of work.
As for aligning the emitters, I don't see any real optical specifications, but given that they're LED and not laser, how sensitive to alignment are they really going to be? Surely the spot size is going to be pretty big by the time it hits a wall 100' away?