2: Use the KEY_DOWN array to tell you if key 1 AND key 2 are both pressed (their value would both be 1), then perform your action. key 1 and key 2 are referred to by the indices (indexes?) in the KEY_DOWN array.
$new_note_id := play_note ([whatever note you want to play - see manual])
3: In the on note event look for the pressing of the trigger key, then set a flag variable. While flag is equal to 1 make it so other on note events will always ignore_event and exit. When the process finishes reset the flag to 0. You would basically be setting the flag to 0 at the end of the on note event that is performing the process. Remember that you can have many on note events occurring simultaneously.