I've just taken a look at 12.0 and 12.4 and I'm afraid I agree with your findings. It is not possible to sort the field list by clicking on column headings in those versions.
I know it doesn't help you, but for the benefit of other readers, I'm pleased to say that the feature is back in the latest version 14.
The only reliable way I can see of doing this in V12 is to copy the rows, one at a time, by clicking on the blank cell to the left of the row in the field list and pasting it in to Notepad. This will eventually create a tab delimited file that you can open back up as a database in Monarch.
If you are using the old .xmod format, then you could potentially open a copy of that file in Monarch and create a model to extract the required data. You will need to rename the copied xmod file to a .txt file so that Monarch will open it as a report instead of a model.