Not sure what you mean by #22, cause if you mean TP22 that's Ground, not power.
This should only be a 3 wire install from the 555.
1 - You would use TP5 for power, VCC.
2 - Then use TP2 or TP22 for Ground.
You don't need any button or switch for power because TP5 is a switched power source on the CG version board.
3 - As for the A button line, you'll have to look at the pics in the first page of this thread for that. About the only Via to use is on the top side of the board. The output of the 555 would go to one side of your new button, the other side would go to the A button line. When the controller is on the 555 is going to run all the time, but it's not going to R/F the A button line until that new button is pressed.
You may need to put an NPN Transistor on the output of that 555 also, to invert the signal, not sure if it will go lo enough to work as a button press, but you can just wire it up as described and see first.