When I suggested to create custom molds that went deeper into the canal to prevent occlusion effects, my audiologist at the university hospital pressed me to get a separate REM measurement for those molds. Apparently, the REM values are strongly dependent on the canal volume between the ear drum and the dome.
So yes, there could be an effect due to the change of dome positiion. However, the effect is small when the change in ear canal volume is small.
Standing waves or vibrations can be diminished by changing the ear canal properties: The dome is the easiest to change. Change the size of the vent, the type of the material. If you suffer no problems with the HA in its proper position, you could try to make sure it doesn’t move. For BTE’s it is enough to slightly change the angle of the flexwires toward the head. This can be done either by changing the position of the connection to the HA in the case of flexwires or heating them with a heat gun.*
*If you want to DIY: The plastic is thermoplastic. So be conservative: Even if it takes 3 minutes to heat it, it’s better than melting!! If you have a spare wire, practice first.