Second, on The 12, if you have a $25 cash balance, the next monthly renewal will come out of your cash balance, even if you have a The 12 PIN stacked. so, you would need to only use $10 PINs for overages on The 12. There is at least one dealer that offers auto-pay for the monthly PINs PLUS on-demand replenishment for your cash balance, so you could set it up so that if your cash balance ever reaches $1.99, an new $10 PIN would be automatically added by the next day.
The other option besides auto-pay PLUS on-demand replenishment would be to use $80 PINs. Your The 12 plan would renew out of that $80 PIN for 6 months, assuming only a very small amount of overages or roaming. But you'd have to mark your calendar AND keep an eye on your balance if you go with the $80 PIN option. The first option of auto-pay PLUS on-demand replenishment is best for your situation, in my opinion
Regarding the expiration of the cash balance when you are on a monthly plan, it would never expire, because every time you buy and add a new monthly PIN, the Standard Plan PIN expiration date is pushed out 30 days, keeping the expiration date at 120 days out.