New 2020 ipad pro 11” case

See here: Compare iPads - Apple.com

Actually size wize they can fit, the dimensions are exactly the same between the first gen and second gen iPad Pro 11".

However given the camera layout on the new iPads, the cutouts on the cases will be different and as such the camera will be covered up if you try to use first gen cases on second gen iPad Pros.

Unknown - you will need to verify compatibility of any case designed for the 2020 iPad Pro with older models.

Yes. Same measurements on the 2018 and 2020 iPad Pro 11” but cameras on back are different sizes.

That is true.