You've probably caused the gearbox to pressurise, which would either burst a gasket, or pop an overpressure valve that may have been designed into the box. Borg-Warner transmissions usually have this. If not, you may have blown the seals on the output shaft of the gearbox, which once stressed, will gradually start to wear until you have a worn out shaft seal and no gearbox oil

You may well have caused some overheating of the oil, which could affect it's properties, so a box oil change might be a good idea.

The torque convertor may well have sustained some damage to it's impeller vanes; this would manifest itself as inefficiency in drive (sounds like the "clutch is slipping"), sluggishness changing gear while accelerating, or "slamming" into gear when the change is being made. Over time, you might even get a vane failure which would be the end of your driving experience that day.

I am surprised that you were able to transit from drive into reverse; most automatics have got some kind of detent mechanism to stop you doing this without conscious choice; given the amount of energy that would have to be dispersed in effectively using your gearbox as a brake for your car from 40 mph I do not think this was such a good idea. What happened?

I would take the oil leak seriously or you could soon be driving a dry box.