OK, I've had a chance to read the incident and there was a lot of issues to take up regarding it but I will stick to the OP/title question.
How do you lower a second from a reverso in a controlled manner?
Option 1) Don't. Conduct a load transfer to a Munter-hitch, backed up with a prussik on the brake strand. Lower smoothly and happily for hundreds and hundreds of feet. Learning load transfers from the internet/books is quite hard IMO, and is best done hands on with someone who is efficient with it.
Option 2) Convert your Reverso in auto-locking mode to a non-auto-locking re-directed plate. With some trepidation I'll describe the basic process here, starting with a loaded Reverso. Learning Load Transfers might actually be easier. I'll also agree with what Jeff said, practice in a CONTROLLED setting, and I'll add botching this will get someone killed. That being said;
1) Add a prussik backup to your brake strand attached to your belay loop.
2) Using a cordelette tie a prussik on the loaded rope, and create a quick "block & tackle tied of with a Mariner's" to relieve some of tension on the locking Reverso.
3) Unlock, and open, the rope biner of the Reverso, and clip it into the master point, without letting the climbing rope come out of it*
3a) Relock the "rope" carabiner that is now also attached to the master point.
4) Unclip the "anchor" carabiner from the reverso, so it is not longer in auto-locking mode, remove that locker if desired.
5) Run the brake strand up through a locking carabiner anywhere higher in the anchor, preferable "the bench" or a bomber piece.
6) Remove the Mariner block and tackle, and lower, with a prussik back-up still in place on the brake strand.
*While this might seem like a potential catastropic error the climber is still held by 2 prussiks at this point, so it is considered redundant.
While this works incredibly smooth when well practiced you can run into issues. Transfering weight from one device to another, i.e. ATC to Gri-Gri, or ATC to Munter, is a very valuable skill for any climber who spends time on multi-pitch trad. If the lead climber can not be lowered to the ground right after a lead fall that resulted in injury, the 2nd must either have the skills to get them down, or wait for help from other parties/rescue.
Self-sufficiency is the best option, as rescue may be hours away. If you multi-picth a lot, sport or trad, you should find someone to teach you these concepts:
1) Belay Escapes
2) Ascending a Loaded Rope
3) Counter-Balance Rappelling
4) Load transfers
5) Belayed lowering
These should not be considered "guide" techniques, or just techniques that rescue personnel should use, but techniques that ANY multi-pitch climber, even someone who doesn't lead, should have a firm understanding of. Climbing is damn fun, so it's hard to dedicate effort/time to this worse case scenarios. We hope to never use them. But should you be locking off a rope as your partner whips high above, and doesn't respond to your calls, "Are you ok?", wouldn't it be nice to know what to do?