The sender (the sender's service provider or organization) decides when to use TLS while the message bounces around internally, then (this is the typical long hop) the recipient's server and the sender's SMTP client negotiate, and then the recipient gets to decide.

The sender may have opinions, but as soon as the message is handed over to the recipient's server, that's it. That's the end of the sender's influence, and that's how it has to be on a global internet. People can connect without having their infrastructure approved by anyone else.

No, it can't. The receiver could be using a non secure SMTP server.

Yes, as well as PGP or S/MIME. It just depends or your relay configuration. We've implemented it switch able. We've customers that are bouncing non PGP encrypted E-Mails.