I'm getting a little confused here, so i'm going to back up and try to
simplify.  There are two distinct (yet somehow overlapping) variables:

 - message-forward-included-headers (an "allowlist")
 - message-forward-ignored-headers (a "blocklist")

Can we agree on that?

Either or both (or neither) of these variables might be applied in any
of the three scenarios.  If both are applied, i have no idea which order
they are applied in, so applying both is probably not a great idea. ☹

The simplest approach would be the following:

On Mon 2021-01-18 16:50:10 +0100, Lars Ingebrigtsen wrote:
> 1) Forward as non-MIME

apply the allowlist.

> 2) MIME, but `message-forward-show-mml'

apply the blocklist.

> 3) MIME, but not `message-forward-show-mml'

apply the blocklist.

What are the downsides to approaching it this way?

     --dkg