well, your convertible top symptom is a little strange, since I guess that takes its cue from the shifter position via the neutral safety switch. Losing the transmission while you're driving along happily eliminates the shifter. Strange combination - maybe there's a coincidence, or maybe the signal is from elsewhere. (or maybe the SL has some shifter arrangement I don't have a clue about). Either way, fully check out the shifter and linkage before tossing $ at the tranny.
Losing all gears with full fluid sounds to me most likely the torque converter. Otherwise, broken input shaft, but I think you would have had some noise, plus it went at least somewhat gradually.

__________________
Prost!