I watched the Mearsheimer lecture. The first point is that he gave it in 2015, so it's pretty old. Second point is that his main idea about Putin's motives are largely falsified by the recent invasion. Mearsheimer says that Putin doesn't want to annex Ukraine, then Putin invades and from his rhetoric it's clear he plans to annex it. So Mearsheimer is probably wrong.

Also, his explanation of the divisions in Ukraine are out of date. Look at the map of the most recent elections. It doesn't follow the usual east-west divide the Mearsheimer suggested. The current government is genuinely popular amongst a wide swath of the population. That was a pro-EU candidate.

Another issue with Mearsheimer's analysis is that Putin's policies in the Donbas doesn't align with the idea that Putin wants a neutral Ukraine (or just wants to "reck" it.) His demands were that Ukraine give the rebel areas autonomy which would give him direct influence in Ukrainian politics.

But all of that is sort of irrelevant because the only reason to discuss Mearsheimer's theory now is to give Russia cover for its current invasion. An invasion which clearly had different motives based on Putin's own justifications ("de-nazification" among others). And, of course, none of this great power politics discussion takes into account what the Ukrainians themselves want. It's not the US's right to tell Ukraine to be a neutral border state and they can't make them a neutral border state if they want. Ukraine has a long history of being a border state--the word "Ukraine" literally means borderland--but that history is one of repeated subjugation by more powerful neighbors. The idea that Russia has more right to security than Ukraine is wild to me and the idea that we even can make Ukraine act according to Mearsheimer's neat little scheme is wrong.