It doesn't contradict SA since they don't mention bonus action casting in their response. They tend to do that alot. Give very curt answers that raise more questions rather than explain the ruling. It is an edge case but does pop up on occasion. It can even be used to a characters, or NPCs, advantage once you are aware of it.
All the scenario's you presented are correct.
The best way to think about the casting limitation is to really phrase it as a rule about 'Bonus Action Spells' and not about spell casting in general. That is why a PC with Action Surge could actually cast two standard action spells in the same turn (given they don't cast a bonus action spell that turn), plus a reaction spell. You could even cast more as a wild magic sorcerer if you get a surge that allows you to take an additional action immediately (81-82 on the table). Theoretically, there is no limit to the number of spells you can cast a turn (if you continue to surge) assuming you DON'T cast a bonus action spell.