This is a fun concept- I was thinking about your six locks and getting the keys when I had a thought. What if you had an alternative means of collecting keys. What if you could hit a target (something easy but that you would not necessairly randomly hit from rebounds etc) that started a pick the lock mode. You could make that as difficult as getting a key by tuning how it worked.

My idea is the mode starts with all targets but two (randomly selected every time) lot up. Then- every time you hit a target it switches the light status- if you are able to turn on ALL lights you pick the lock. If you keep hitting lit targets it gets harder and harder as they turn off when you hit them. Timed mode-

perhaps even make it so that in order to enter mode you need to do some rapid flipper sequence within 1 second of hitting the pick the lock mode target- that way you can choose to enter mode.

Anyhow- not sure this is even consistent with your idea for play- but seemed fun to me so wanted to share.