Peace be with you!
Debating schismatics is even harder than debating Protestants, often because the items of debate tend to be a lot of very small details. I could give you my complete disproving of sedevacantism right now, but not only would that be much too long, it is still on the list of banned topics.
I would rather keep a debate with SSPX folks to these lines: either they believe in the infallibility of the pope (and they are not sedevacantists, so they do claim allegence to Benedict XVI–their website says so) or they don’t and they are heretics for denying a mandatory article of the faith. Also, whether they believe in the infallibility of Church councils, as VII was one such. They can’t have things both ways. Either they agree completely with what the Church teaches, or they are not Catholic. You are not the one who needs to defend yourself to them…they’re the schismatics.
P.S. Here is evidence of their ridiculous arguments: on their website they claim that a “table” is used in the novus ordo instead of an altar. Would they be willing to tell Benedict XVI that the high altar at St. Peter’s basilica is simply a table? They also claim that reception of communion under both forms is a liturgical abuse…an abuse! Absurd! They also try to say that communion is now referred to as a meal rather than a sacrifice. Funny, every mass I’ve ever been to (even a horrid Protestant-like youth mass) has made it clear that what is occuring is a sacrifice. These people are full of it.