PAPR (Powered air purifying respirator) power regulation and low battery warning

[Axiverse]

I'm looking at making some PAPRs for some friends in the medical field. In principle it's quite simple, a 12v fans connected to some 18650 batteries. I will add a buck boost regulator to stabilize the voltage from the batteries to the 12v expected by the fans.

I have two things I'm trying to figure out.

1. How do I add some kind of warning light. I'm thinking about putting 4 16850 batteries in series directly and use that into the voltage regulator. I haven't found any modules that will allow me to warn on low voltage on batteries in series. Any ideas what the easiest way to do this is? I have found 1-8s voltage alarms, but those are per cell. I'm not sure how to have the alarm hooked up and have it output 14.7v combined. The regular that I have in mind requires 8V minimum.
2. Do I have to do anything special to regulate the lithium batteries? I know they may discharge at different rates and there is logic in the charging, but is it safe to directly put them in series? How if I wanted longer battery life? Can I put 3 sets of 4 batteries in parallel?

Thanks!