I’m aware I can publish events that can be read using the SDK on Android however is there a way to do the reverse; read an Android Variable (set value - probably of type double) and use it to act upon within the actual firmware?
You’d call a Particle.function() via the respective POST request or send an event to which the Photon has subscribed.
In both cases your data will be represented as string in the payload.
With the Particle APIs the Photon “never” pulls/requests data fom the Particle cloud directly - it always gets data sent or sends data (aka Particle.publish()).