What are you talking about? The 6th perek is an addition, as it begins, שנו חכמים בלשון המשנה. That is not true for the 5th, which is a bona fide, legitimate mishna. As with much of Pirkei Avos, the obligations contained are not strict halacha, but a guide. I am not going to obligate a boy to marry at 18 based on the mishna. But nor will I consider the mishna being followed when someone who is ready for marriage and postpones it for an extended time. I referenced the quote coming from the fifth perek for the obvious reason, but you decided to misrepresent it.
As for interrupting studies, there are guidelines of when one must stop learning for various mitzvos. That also includes an issur to learn lest one fail to fulfill the mitzvah on time. Granted, we are not dealing with either minutes or hours here, but rather years. For a serious learner, your argument would be that he should avoid marriage altogether, a la Ben Azai. I reject that suggestion, as there has not been anyone alive in our generations and many in the past who could be a Ben Azai. So it is about an individual judgment of when. Accepted. But the lengths you suggest seem grandiose.