Okay, this answers part of the question:
Example 1: Taxpayer A has disallowed PALs of $30,000 and $50,000 for 2018 and 2017, respectively, and has passive income of $35,000 in 2019. All income and losses are from qualified trades or businesses. What is QBI? ...In Example 1, QBI would equal the full $35,000 in 2019. The 2017 PAL is the older previously disallowed PAL and is otherwise allowed against passive income in the current tax year. However, because it is carried over from a pre-2018 tax year, it is disregarded for purposes of determining QBI.
https://www.thetaxadviser.com/issues/2019/dec/sec-199-update.html
...But here's a kink: What if Taxpayer A also had $50,000 in QBI loss carryforward from 2017? In this case, it would seem the QBI is zero in 2019. Can anyone confirm?
