I see what you mean…
But as far as I found, in reality DC making a standard as email messenger and while it still Beta, I think other MUA who willing to implement encrypted communication using DC protocol would benefit using Bcc: and parse encrypted mail headers either as part of memoryhole or my preference would be: “Wrap the whole message as an message/rfc822 sub-part” and “Only use embedded text/rfc822-headers parts”.
I think if mutual p2p encryption have been established then subsequent communication should use encrypted email header only regardless if it is a group or p2p. I think also that it isn’t a good idea to combine in group members, who aren’t established encrypted channels otherwise it would be a mess for sure regardless of my proposal.