Noel,
a) provides no quoting context
That's true. It has been discussed in beta@ but is difficult given that various email user interfaces behave differently with mailto: links.
Some don't support a "body" parameter (which would be needed to provide a quote of the original), and even where that is supported it could only be the whole original message - or a predetermined snippet of it - there's no mechanism to tell Thunderbird (or any other email interface) to place selected text from the current message into the new message composition.
b) uses the wrong (default) persona for the reply
That is also true. The mailto: link functions as a "New Message" command, not a "Reply" command. So there is no context Thunderbird can use to apply the correct persona; the user must manually correct the "From" address in the message composition window.
The original user was replying to a digest so the option to reply
directly will not be available.
Correct.
The user's other option is to use the View/Reply Online link instead, and compose the reply using the group's web page rather than Thunderbird.
Shal