The p20pro is slightly better in low light and telephoto but dont have the ultra wide angle.
I would choose the p20pro over the m20pro if budget is a constrain but would choose the m20 over the p20pro ,just for the kick of that ultra wide feature.

Just tried low light shots for both phones during lunch today.. low light on Mate 20 Pro is much superior than Mate 20.. comparing to P20 Pro I must say Mate 20 Pro is not far behind if not on par..

So far so good on mine which setup yesterday and start using today. Are your things due to setup issue or it is really a phone hardware issue?

Go sc and request 1 to 1 due to they can't fix your phone.
That's your right to request 1 to 1.

Your device is mate 20. Non X, Non Pro?

That's bad, I think you can try one SC and see what's their respond, if no good then change to another SC until you demand them to change a new set for you. Guess that's the only way.

I only know that when thing like this happen, you have to make your stand and raise up volume, only they will "hear" you.

You can try make appointments for 1 to 1.
So you can instant change on the date. Where how I do for my m20p