Upon further inspection, here's a possible answer to Issue 1) in the OP.
The 2019 1040 had a Line 11b., Taxable Income. I suspect, but cannot confirm, that the problem may simply be that the field description on the "Let's get some info from your 2020 tax return" screen was not updated from last year, is now mislabeled, and should say "Taxable Income (Form 1040, line 15)". That's the Taxable Income line on both the 2020 standard 1040 and the 1040-SR. The $0.00 prefilled in my case is consistent with my 0.00 amount on that 2020 Line 15 (lucky me).
I would like a TurboTax rep to confirm my suspicion or explain that something else is going on. If my suspicion is correct, then a fix to that screen is in order.
