What are the chances it’s backwards compatible with Switch games? Digital and physical?

100.0%. There is just no benefit whatsoever to rocking that boat. The Switch is a converted Android gizmo, ARM is still the only sane answer for high-performance mobile platforms, and their video hardware is from the company that bought ARM. To say nothing of the PR shitstorm it would be to slice their market in half.

That said, you’ll still have to buy Virtual Console games separately, because Nintendo.