1.  The canon was codified by Pope Gregory the Great in the 600s, so the canon is not 1,700 years old but only 1,400.
2.  The canon contains names of martyrs, which did not come from Christ (because the martyrs weren't alive when Christ said the first Mass) but from the pope.
3.  What one pope can add, another can subtract.  The papacy has the power to "bind and loose".  A bad pope can do bad things; a bad pope is a punishment for sins.
4.  The resignation of popes and the canon law of this situation is highly technical and not at all comparable to the liturgy.  I have no idea.
5.  Most everything the modernists do is odd, because they aren't orthodox and they hate catholicism.  But that doesn't mean they can't do such things.  They spend all their time reading the fine print and looking for loopholes with which to create confusion and doubt, because God will not allow them to do any lasting damage.  All of the changes of the last 100 years will be rectified in the future and the glories of the Church will return soon enough.  1,960 years of orthodoxy vs 50 years of confusion.  Have patience!  God will not be mocked forever!