"why he opposed Archbishop Lefebvre so forcefully for defending the traditional Mass and the traditional priesthood"
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Archbishop Lefebvre was told not to pursue with new ordinations for the time of Vatican's investigation. But he was much like you, he knew better than the Pope. So he got exactly what he asked for.
No, your interpretation is simplistic. The Pope wished to wipe out Traditional Catholicism by starving it of traditional priests--which meant denying the Archbishop the right to ordain traditional bishops. Only bishops, after all, can ordain priests. If traditionalism had no bishops--the Archbishop himself was 82 years old and ailing and there were no longer any other traditional seminaries anywhere--who would ordain traditional priests? And without such priests, how could traditional Catholicism survive? So the stakes were far higher than you suppose. The whole of Catholic Tradition was at stake. The Archbishop was obliged to disobey--in order to save the Catholic faith.
What's more he was covered by Canon Law itself, which the Pope failed to mention in his motu proprio. Canon 1323 specifically excepted from punishment anyone who acted out of a state of necessity. The Archbishop evoked this law--properly and legally. What's more, still another canon provided that even if the individual were MISTAKEN about such a state, the penalty would not be incurred. As long as the individual sincerely believed there was an emergency, no penalty could be incurred. So the Archbishop was innocent--despite what the Pope said. There was no excommunication--and there certainly was no schism. And remember--Canon Law itself is the Pope's own law. It is a far more important document than the Ecclesia Dei letter.