This is strictly a personal opinion, but from what I know of Mgr Lefebvre, he wasn’t all that much of a “hardliner.” One has to keep in mind that in the early days of the SSPX, he did everything strictly “by the book” even to the point of having the then-bishop of Fribourg sponsor the society in its initial stage. It seems to me the biggest problem at the time was the times: it was the 1970s and unfortunately, Mgr Lefebvre’s best efforts seem to have been thwarted at every turn. On the one hand, there were those in power in Rome who had their own agenda and, simply put, there was no room on it for the likes of Mgr Lefebvre. They, of course, did everything in their considerable power to sway Paul VI. On the other hand, there were the real “hardliners” whose agenda had no room for anything to do with the Council, and who joined-up with the SSPX. For their part, they did everything they could to sway Mgr Lefebvre in their direction.
After Paul VI went to his eternal rest, Mgr Lefebvre kept on his path which, of course, led to the SSPV split. And then came the 1988 debacle, which seems to me to have been in main a battle of wills of two very stubborn parties. And then Mgr Lefebvre went to his eternal rest.
As I mentioned on another thread recently, I often wonder if, in 1988, Mgr Lefebvre had been manipulated by some in the SSPX having their own ambitions. I don’t claim to know the answer, but if’s true it would explain a lot. My impression is that the SSPX has become increasingly hard-line since the passing of Mgr Lefebvre. Now, of course, we have PP Benedict XVI who is making a good-faith effort, but things have changed a lot within the SSPX.