So seems like there may be two different, but extremely related isssues going on. I cannot report a bug, because when I tried the url given in this thread, it is apparently for developers only. If any of you developers can reproduce my issue, it would be wonderful if you could submit a bug report on our behalf! Thanks. I sure hope they come up with a fix for both of our issues. I can feel your frustrations.
Done! Bug 12038482
Summary:
In previous versions, if you create a new message or a reply, while looking at a message, the default sender of the new message would be the account on which the message being viewed was created. Now, the default sender for new messages is always the account of the mailbox selected on the left panel. When the mailbox selected is a user-created folder, or is the "uber-inbox", then the default is always the first inbox in the list.
Steps to Reproduce:
Create two accounts, A and B.
Receive mail on both accounts.
Make sure the "uber-inbox" is selected, not any individual mailbox
View a message received on account A, create a new message.
View a message received on account B, create a new message.
Expected Results:
The new message created when viewing a message from account A should be from account A.
The new message created when viewing a message from account B should be from account B.
Actual Results:
Both message will be from the same account (whichever is the first inbox in the uber-inbox, apparently).
Regression:
Lion was smart enough to look at the currently selected message. Mountain Lion ignores the current message and only looks at the current mailbox.
Notes:
The behavior when replying is correct. It is only when creating a new message, that the defect arises.
This bug is similar to 12025786, however it differs in that in that other report, even replies do not select the correct account.