Benedict would know if "for all" made it invalid DESPITE the intention of the priest.
Well, he may not know personally, but the Vatican theologians could study the matter. The point is, even if he knew, he's never made a statement on the issue (nor did Paul VI). All Benedict said was that "for many" was the correct translation. This doesn't mean that if a "priest" uses "for many" that the new mass is valid. It just means that this one, particular invalidating aspect is no longer a problem. The remaining potentially-invalidating aspects still remain.