The latest update allows searching for usernames, but they cannot automatically be copied back over from the popup search window (i.e. the user must copy them by hand).
To fix the tab bug, there were two options: one was to autofocus the newly added user field, while still not being able to tab between them. The other was to disable tabindexing entirely and force the user to tab through all of the smilies, bbcode, etc. I chose the former as the less of two evils, although it would be trivial to modify for the latter.
The last bug is really big one that needs to be fixed before release, if anyone could provide any pointers on this I'd appreciate it.
ACP control is coming soon.