Other bits and peaces, if I may.

Question

Three points to get out of my system, to more appreciate things the way they are now. For Thunderbird. So you have these thoughts at the ready to make a choice!)
• Selecting two (or more) emails to combine in a reply doesn't work, it makes the reply button disappear (to make room for other useful buttons). Or just melting emails (combining), seems to be a good idea to me. (Doesn't have to be a two file read into one though, creating one inbox email; could be just linked for that purpose, perhaps in combination with a 'defragged' state.)
• I know this trick to shield a mailing list is to put all the recipient email addresses in the BCC field, but isn't it better to put it all in the To field, with a [ ] checkbox next to it (plus descriptive icon, mouse-over popup info), to do it that way ánd (optionally) create an mailing list this way? I think that would be intuitive and easy.
• Also, sometimes you see all email addresses already summed up in the email (through past replying); I think it should regex-onally (regex wise, with regex) be removed. (Using regex, standard, automatically to help protect that information, ánd clean up the email a little.)
• I wonder if I can create an email address with a ♪. I'll try it when I need to. Would it solve anything? Perhaps not, other than it's fun!

Answer 1

''re : Selecting two (or more) emails to combine in a reply''
A Reply means to return a response to that specific email.
So, it is impossible to Reply to one email from person 'A' and include another email from person 'B' as a Reply because the conversation is with 'A' not 'B'.
However, you can always 'Reply' to 'A' and include 'B's email address as a Cc or Bcc. which is the normal process when wanting to include someone else in the conversation.
Alternatively, Reply to 'A' and then Reply to B and attach the response email you sent to 'A' as an attachment.
''re: Also, sometimes you see all email addresses already summed up in the email (through past replying);''
You must have different settings in your 'Reply' because all I see is:
* eg: On 9/12/14 01:01 PM, name of person wrote:
there are no email addresses.
But, if this email you want to reply to was previously replied by the sender and you are now replying to it, their system may have included an email address, so it could appear in the Reply you are making because it is part of the email you received.
In which case you could manually alter the email content.
If you only want to reply to a section of the email, highlight that section of content and then click on 'Reply'.
However, If I 'Forward' an email then email addresses of that forwarded email are included, because the person you are sending to may need to know where the original email came from.
In which case either edit the email addresses to remove or use the method of selecting the content first and then click on 'Forward'.
Unfortunately, some people do not edit emails before forwarding so end up forwarding a load of other peoples email addresses.
It is the responsibility of the sender to consider what is being sent and modify appropriately.