For my 92 W124, the rotating mechanism goes with the armrest, it is embedded into the plastic frame, the seat has only 2 mounting bolts to mount armrest, I'll just assume your 126 has the same armrest as my 124.
I guess in your case, RHD, you want to move the armrest to the driver's seat, you can move the mounting bolts from the seat, but the due the rotation direction of the armrest, it should be always mounted to the right side of the seat, which in your case may not have enough room and you have armrest on the door already.
If you want to mount it onto the LHS of the seat, the armrest will have to sway from horizontal position downward, you may not have enough room for this motion.
Please forgive my crappy Engilish, hope I answered your question.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.