I have seen controllers that use low voltage for brake sensors, typically ≤1 volt but from my experience they are not common, it's typically either ground or 12v. The easiest solution that I can think of is to use a low trigger relay to activate the brake lights, if you don't know what they are it's a relay with a signal input, when the signal input receives 0v or ground, the relay gets activated which in your case it would be perfect because when activating the lever, the 2.8v pulls to zero. However those relays commonly require 12v to work which would require you to get a dc-dc buck converter, you can get some that are really small like this one:
View attachment 332694
I can think of other solutions to activating the brake light but they all require 12v and the low trigger relay is the simplest one. I don't think there's anything you can do with that 2.8v especially since it pulls to zero when activated.
Regarding the brake light wiring, I think it doesn't share a common ground probably to increase the compatibility, some systems use ground to activate brake lights, some positive input so no matter what system the consumer is using, they will be able to use the brake light. If the brake light had a common ground and a bike used ground to activate the brake light then non of the lights (running light and indicators) would work unless the brake lights are activated which would be silly.