I'm saying how does the current that is now buzzing around the metal frame/body make its way back to the RCD?
What "current buzzing around the frame" ?
Are you familiar with
Kirchoff's 1st (or Current) Law ? Put simply, all the current going into/out of the device (via whatever routes) must sum to zero. If there is no path to ether from the frame (lets say it's sat on a perfectly insulating surface and has no earth connection) then there can be no flow of current between frame and earth. If the live supply wire comes free from it's terminal and touches the frame then two things happen :
1) The supply current will stop. There's no longer a path from live to neutral via the device load, and no path to earth from the frame, therefore when you apply Kirchoff's law, there can be no current in the live. No current in the neutral, no current in the live, therefore no current imbalance and the RCD will not trip.
2) The frame will become live. Simply, there's no a connection from the live supply to the frame, so the frame will be sat there at mains live voltage.
......because the point is if we say the metal frame is not earthed AND we say there is no imbalance in current (meaning the RCD won't trip), then that must mean the current is making its way back via the circuit's neutral wire....correct?
No, as above, there will be no supply current - it has nowhere to go to.
Remember, the supply does not "push current" through the loads, it forces a voltage onto the loads, the current is drawn according to the supply voltage and load characteristics. That's a common misunderstanding - people thing that you cannot use (say) a 2 amp power supply to for a device that only draws (say) 1/2 amp, they believe that somehow the 2A supplyt will force 4 times the current through the device and fry it. Assuming the voltage is the same, the device will simply draw it's normal 1/4A from the supply.
Now, there can be a discussion as to whether a large device sat on an insulating mat really will pass zero current to earth. In practice, the frame, mat, and floor/ground will form a small capacitance which in the presence of an AC voltage will pass a very small current. But for this discussion that current will be small enough to ignore.