Yes to all three questions! It is hard to configure, though.
Personally I would like to see some of Deltachat’s standardization spread. Free Chatmail accounts and SecureJoin-standard QR key exchange would be great features for any mailclient to implement.
While Deltachat clearIy started with such interoperability ambitions, I think many of the devs have gotten discouraged. Even the Autocrypt standard has seen little adoption. E-mail is increasingly concentrated in the hands of a few oligopolist providers, who increasingly only reliably accept mail from one another, rejecting private servers at the slightest excuse. Additional protocols have made running a mailserver ever harder (there is active work on multiple easy-setup Chatmail server implementations).
But I think this situation, and Deltachat’s increasing adoption, actually improve our chances of getting other mailclients onboard.