I thought the answers to your question were very conclusive. You're over=thinking this. The Volvo OHC valve train is just like that of most cars today like Toyota, Lexus, Mazda, BMW, Mercedes, Nissan, etc. None of them are called flat tappet engines. Flat tappet means a flat bottom lifter on a pushrod engine. Valve springs on pushrod engines are stiffer to control the extra weight. The flat bottomed lifter used on those engines requires an additive to prevent wear. Roller lifter pushrod engines use valve springs that are even stiffer because rollers weigh even more but there is no lubrication problem.
So there you have it. You can use a modern oil without or little, ZDDP.
Your theory about cats being used since the 1970's doesn't hold water. Early cats were tolerant of the additive. The latest generation of cats are materially different, and it's only those ones that are poisoned by ZDDP.
So you're arrived at the right conclusion. Use a modern SN rated oil.