Sasha Thank you for sharing!
You can't punish your mom because she's your mom. If she does cheat on the belt you should have a conversation with her about what she did, how she feels about it, and how she would like to proceed.
You could buy the belt with D-rings; such that she is reminded that additional measures are possible; but again you can't actually punish her, that has to be something that she wants.
Have you decided what type of belt to purchase and if she is already at the point where she can go overnight isn't this enough of a commitment to buy a belt now?
How do you feel about being mutual key holders? Has it brought you closer?