@fgor, it sounds like you had a great bootfitter and your boot probably fits as well as it can. Congrats! My guess would be that if you test the boots with skis on and a friend twisting the skis, there will be no "loose steering wheel" effect. So maybe boot fit had nothing to do with it.
Discomfort with the wobble is natural the first time you notice it. Those 88s with a 13m TR will give you some wobble, for sure. Simply glancing down may have been the trigger to the fall. Or whatever caused one ski to not wobble while the other did, who knows....
Go to safer terrain and ski straight runs over and over to gain stability on flat skis. You can try getting into a tuck for the extra stability that comes from lowering the body. Look up low tuck and high tuck for proper body positioning. Close your ankles to get your knees as far forward as possible, and shoot your hands waaay forward for more weight over the shovels, and you'll be good as gold. Make straight runs. You'll feel the wobble in both skis, or should. Get used to equalizing your weight distribution between the skis which should keep them wobbling equally. Start with a low tuck, then a high tuck, then work on standing up while going straight, again on the same safe terrain. Avoid looking down at the skis. Once you feel like you know what those skis want to do and know you can ride them, take it back up to that ridge.
Note: you'll go fast in a low tuck. The wind resistance will be minimized, so choose beginner/low pitch areas to work on this. You'll need low traffic areas and good peripheral vision to avoid running into folks. Be careful.