Well, since even knowledgable Trads acknowledge there is no certainty except for SPPX ordained priests I should avoid the FSSP?

I'm no authority so all I will tell you is that I will never go to or use the FSSP for anything because I have doubts, not only about their priests' validity, but also I do not want to expose myself to their preaching. I also would not go to some valid trad priests because I do not want to expose myself to their preaching, but unlike many, I do not have to concern myself with those worries - and Deo Gratias for that brother!

For me, I do not have your worries, I have gone to many different independent priests in my life, and Deo Gratias I have access to and have been going to SSPX since before they were the SSPX. I use the SSPX for my Mass and sacraments and know they are valid priests.