While we are on the topic, I can share a couple other related issues I have noticed with the editor.
At times I will make a post, it will appear perfectly, then I will edit the post and save, and I will notice that the line spaces changed elsewhere in the post, in an area I did not touch. This has happened on multiple occasions. If you do go digging in that dark evil place, you may notice something related to this issue.
Also, in the regular editor, when images are inserted they appear where ever the cursor has been placed. When we switch to the Rich Text editor, images are always inserted at the bottom.
I did look at the loss of tabs in Webkit, and it doesn't look like the span that has the tab is even sent to the server. I might be able to change that, though I'm not sure what in TinyMCE is causing that.
Regarding the line break handling, I've basically rewritten a good portion of our HTML to BB code translation and it seems much more accurate. It's running here, so let me know if you still see any oddities. (Note, this only changes what happens when a post is saved, not the translation that has to be done if you try to edit it again. There are still some tricky issues with that.)