Connecting Common Wire on Air Handler

Hi everyone. I am trying to connect the Common wire to my Nest thermostat, so it charges the battery. Fortunately, the therm wire has a 5th wire running from therm to the air handler. It's currently disconnected at the therm. What is confusing me is that this 5th blue wire is connected to the R terminal on the air handler.

There is a second wire also connected to the air handler's R terminal, which travels a bit and then connects to the Rh line from thermostat and the R from the boiler at a wire nut.

Outline of the Thermostat Wiring

G (therm) connects to G (air handler)
Rh (therm) connects to R (air handler) and R (boiler). These three Rs connect in the 'junction' photo I've linked to below.
C (therm) was connected to the R (air handler); I disconnected the side at the thermostat; other side is still at R on air handler
W (therm) <--> W (boiler)
Y (therm) <--> Condensation pump's Red. Condensation pump's White goes offscreen to A/C compressor Red. Compressor's white connects to air handler's Common terminal. From what I understand, this is done so that if condensate tank overfills, 24vac is removed from the A/C compressor.

(I'll post a link to photos of the wiring in a minute.)

I've been cautioned that this situation is not optimal because the boiler R and the air handler's R are connected to each other and go to a single Rh at the thermostat. That's worked well because there isn't a Common.

So the main question is how to determine whether I can disconnect the Blue 5th wire that runs to the thermostat from the air handler's R terminal, and move it to the C terminal. Then attach the other side to the C terminal on the Nest. Or would this compromise the systems because the boiler and air handler would now both be connected at the C and R.

Thanks so much for the help.