Grammar is not logic. There is no grammatical rule requiring that "to" be repeated in a series of infinitives after the initial marker. Nor is there a grammatical rule requiring that "to" be omitted in such a series after the initial marker. Consequently, the presence or absence of "to" after the initial infinitival marker has no semantic meaning.
"I do not like to play basketball or eat spiders" and "I do not like to play basketball or to eat spiders" are both grammatical and have identical meanings. There is no implication of temporal sequence. Neither sentence implies that I may like eating spiders if I have not previously played basketball.
I agree with those saying that if you mean that historians find it difficult to argue the effect of historical events on society because they find it difficult to isolate events, you need to do a lot more than worry about whether "to" can be omitted from "to argue" if it is preceded by "to isolate."
But I suspect that your worry really stems from the fact that the thought being expressed is muddy. It is not that historians are unique in finding it difficult to isolate historical events. Historical events are never isolated. Impossibility is not a mere difficulty, and the failure of historians to do the impossible is not a peculiarity of historians. Moreover, historians frequently do present arguments about the effects of history on society without any apparent difficulty. That part of your thought is obviously wrong factually. Consequently, I suspect that you are trying to say something like
"Because historical events never happen in isolation, historians' assessment of the effect on society of a particular event is wrong/doubtful/uncertain."
Whether or not some or all of those are sound conclusions, they are clear-cut and at least plausible propositions. I have often found that worrying about exactly how to phrase a sentence indicates that the true problem lies not in the sentence's phrasing, but rather that the sentence does not yet reflect a well formed thought.