The 722.6 does have a drain plug, it requires a 5mm allen socket (or possibly 6mm) to remove. Starting about model year 2000 they eliminated the drain plug in the torque converter though.
Drain out ALL the engine oil and start over. If you already replaced the filter, I'd leave the new filter on, drain the OIL pan and put in about 5 qts, total capacity with filter is 5.8 qts. DON'T start the engine until you've done this.
Next I'd go to the dealer and get 3 quarts of the MB ATF for the 722.6, put in 2 of them and start the engine. See if it's moving/shifting OK. At that point I'd get it to the dealer to get the AT fluid level checked.