I could not find what you cited in the Ritva, but I did find it in the Chidushei Rashba Berachos 13a DH MATN' HAYA KOREI (which is cited also by SHULCHAN ARUCH OC 104:6 and REMA there 65:1).

The Rashba there cites the Talmud Yerushalmi Berachos 2:1 which states that it is reasonable to say that one reckons the maximum amount of time which can be waited according to how long it takes each individual to finish, not how long it takes the average reader.

Artscroll here, note 2, explain that the very definition of "Keday Ligmor Es Kulah" (the amount of time it takes to finish it all) is not in fact a fixed number of minutes. Rather the definition is how long it takes each individual. In other words the "Shiur" (measurement of time) is personalized. This means that the definition of the Shiur does not relate to the average person in the world, but rather that for Reuven, by definition, the Shiur is different than for Shimon. Therefore there is no problem of "Im keyn..." because the shiur is how long it takes Reuven. In contrast, if the Shiur would have been the amount of time it takes Reuven to finish if he stopped halfway through the Megilah, this would be a different length of time for Reuven himself than if he stopped at the beginning, and for Reuven himself there would be a different Shiur each time, which represents the problem of "Im keyn....".

(See the Ra'ah Berachos 24b, and note 64 there - cited in note 7 to the Rashba Berachos 13a - who writes that one measures the length of time according to the average person. This appears to disagree with the Rashba, and maintains that one does measure the average number of minutes taken by the average person to read the Megilah).

I found in Chidushim u'Biurim by Rav Chaim Greinemann Shlita, of Bnei Brak, that he explains Rav Yosef"s reply that one measures "Keday Ligmor Es Kulah" from the beginning to the end, in a way that means your question is not difficult. R. Greinemann writes that "Im Keyn..." means that it is illogical that the length of time one may pause without having to read the Megilah again should decrease the nearer one reaches the end of the Megilah.

I understand this to mean that - to take an extreme example - if someone just has the last word of the Megilah ("Zaro") still to read, and he would take a very short break, but slightly longer than it takes to say the word "Zaro", this would mean he lost the entire reading and would have to go all the way back to the beginning and start again. This clearly seems absurd. Therefore Rav Yosef replied that this cannot be the way of measuring the pause, but rather one has to measure right from the beginning to the end.

According to this there is no comparison between the Gemara's case, and between the Din you mentioned that the full length of what is read is calculated according to how long it takes each individual to read it. The way "Im Keyn..." is used by our Gemara at any rate, has nothing to do with saying that the length of time for each individual must be uniform.

PURIM SAMEACH

D. Bloom