My parents have gotten the "grandson is in jail, send bail money" call a few times. They don't send money but do engage with the caller more than I am comfortable with. Afterwards, they are always shaken enough to call and check on the grandkids even though its clearly a scam and they know in their brain, emotionally they worry it might be true.

I've talked with the "send money to pay overdue electric bill or power will be cut off" scammers more than once.

At what point do you realize a person is incapable of making certain decisions on their own?

This is a terribly difficult dilema. Both financially but also with healthcare decisions. The best plan is to get them to agree to giving up some control before it is an issue. "You are fine now but what if something would happen to you?" is easier than "you have to give up some control right now."

first before his shared assets and savings are unfrozen for her.

This is a real problem. Even shared assets can be frozen making the surviving spouse unable to access them for a period of time. My older brother passed away and he had put his wifes name on everything thinking she could just continue as usual when he died. That did not happen. This issue is one that requires a will and estate planning. Depending on your resources, there are lawyers that can make this easy. Or there are online resources that allow you to make a simple (but fully legal and enforceable) will.

These are age old problems and I have offered nothing new. The one recent development I have seen is some companies are offering apps that basically give you "read only" access to your parent finances. It provides a good check that nothing unusual is happening with their finances without forcing them to give up any control. The problem is that it does require them to give up privacy. Many people don't want anyone knowing anything about their finances. How much money they have or how they spend it.