Remove MIME in Message mode when sending just text

[BP25]

Hello,

I use Message mode as opposed to Mail mode because I sometimes send
attachments. However most of the time I just send plain text messages:
then Message mode nonetheless adds the MIME headers and because I make
Rmail not display automatically the plain text part then I have to
manually click RET every time. I don't want to fix this by customising
Rmail more: how to make Message not uselessly add MIME when not required
becaue the message is just plain text?

Also, I have occasionally received some messages which looks
almost-MIME: they only have this header: Content-Type: text/plain;
charset=Utf-8 (these messages are displayed by Rmail
immediately). Shouldn't this header be absent or is this header usually
required also for non-MIME messages?

I also want to say that on these such messages (and on non-MIME
messages) then (rmail-mime) for some reason has the same effect of
(rmail-toggle-header): is this on purpose?

And instead on MIME messages, (rmail-toggle-header) and (rmail-mime)
cannot activate simultaneously but doing one undoes the other: is this
on purpose?

Finally, is it a policy of this mailing list not to reply to the sender
of the messages ("reply to all")? Because I'm not subscribed and I have
to look for replies to my messages on the public archive.