A few questions related to development on Actionable Message.

Question

According to the official documentation I have implemented Actionable Message sending and updating in my application. However, I have a few details that I need to check with MS experts:

1. Assuming that an employee has created a Provider in Actionable Message Developer Dashboard, if the employee leaves, will this Provider remain active? Who has the right to maintain this Provider?
2. In the process of creating a new Provider, there is a field named "Public Key" (see the picture below). What is the field for? I didn't use this Key throughout the subsequent coding process but still successfully sent the actionable message.
3. There is a reference to SignedCard in this documentation page, what are the adverse effects if we don't have a Sign Adaptive Card? (We already use HTTPS.) If you strongly recommend using SignedCard, do you have any sample code in Java? Our backend development environment is Java, but I see only .NET and Node.js sample code in the documentation.