How to extract channels that were accidentally recorded in mono

To add a bit to Richard's reply - the way to achieve this that is easiest (and the way the software was designed to work) is to open the track in Waveform and place markers around each person's contribution as it's made. The process is to highlight just that bit, and hit F8. This will create a marker range that will show up in the marker list, and you can give it a name there as well if you want. In fact you'll want to do that just so you can tell which clip is from which speaker otherwise they'll all lookk the same. When you've completed that, go to the marker list. If you right-click on any one of the ranges you'll get an option to insert it into a multitrack session - either a new one, or one that's already open. It's important to note that the range will be inserted on the track that's highlighted and at the point that the cursor is in the session, which in this case won't exactly help you get the clips back into the correct place, but it's a start and it's easy to slide them around.

If you really don't want to go to all that trouble, then the other option you have is to use the Speech Volume Leveller across the whole of your original file (actually you should do this to a copy, not the original) which will make a pretty reasonable job of getting all of the speech to the same level anyway. That just leaves you with an editing job, which might be quicker.

I would put the interview into a multitrack session  and copy and paste each speaker to a different track. You can then mute the master and do level changes and eq on the new tracks.