The only difference is the colour. SCD953/26 is white and SCD923/26 is grey. SCD971/26 is another white colour.

As I only know the 53, my answer is without guarantee.

Presumably these are the versions and there is no big difference...

But it is important to know that the cam is not compatible with the Philips hue app (which was once mentioned here somewhere) but has a separate app that is quite good.

The tripod for the cam is definitely recommended.
Cheap but pricelessly good.
In my opinion

Unfortunately, I can't answer that. I gave it to parents as a gift for the birth of their child. They are very happy with it.