Rescue helicopter micro with 1S batteries

Arakon is right about the resistor not being the way to go. However, having a 5V signal on a 3V line may be a problem even if you don't route power through the plane locator. May not, either. Lots of people use 6.6V LifePo's instead of 4.8V NiMH or 6V AA packs for Rx packs. Maybe try asking on a general RC forum instead of one devoted to third party firmware.

As to which servo to use - doesn't really matter. I'd take whichever channel you use least when flying, but all of them should be active enough to keep it from going off in the air.

I agree with you that makes resistance a voltage drop depending on the current flowing through it.
I just saw that in fact all the entries and outputs from Lost Plane Finder Alarm are welded together, so it does not process any signal inside, which is even simpler. It is true that there could be a risk of mixing between 3V and 5V but I do not see what else. Here is the updated schematic.
Thank you very much everyone for your help.
Have a good day.