You always have the right to choose where your car is repaired, I would have insisted it go to your Mercedes approved repairer. Regretfully, there is not much you can do about the non availability of parts as that's not the insurer or repairers fault, it is largely down to Covid. Unless you paid for a guaranteed hire car, you are certainly not entitled like for like. All you can do at this time is be patient. Hard to hear, I know, but things are not normal currently and that has to be factored in. You can raise a complaint with the insurer, and advise you feel you were steered into using their repairer and wish you had chosen your own repairer.

If the car cannot be repaired due to non availability or parts, your insurer "could" provide you with a "cash in lieu" of repair to fulfil their contract. This would leave you with a car unrepaired and no parts to repair it, but the insurer WILL have fulfilled it's contract. I suggest caution on your part.