For example during the Great Western Schism. Obviously the antipopes and the people who installed them were schismatic, but did the Church consider the lesser clergy and laypeople in their countries to be schismatic too? Would a random parish priest in France or his parishioners have been damned for schism if they died during the Schism?

St. Vincent Ferrer was a follower of an anti-pope and this was not a bar to his canonization.  This shows that the Church does not consider people automatically condemned to hell for following an antipope.

He abandoned the anti-pope in the end however. 

But there was no guarantee that no one would die before the Great Schism was sorted out. What of those poor people who died before it was sorted out?

It should be common sense that it's not incuмbent upon laymen to figure out the Pope question personally, and act on it CORRECTLY because if they get it wrong, they'll be damned.

That would be ridiculous. Most of the information is either above most laymen (requiring knowledge of theology, Canon Law, etc.), or inaccessible (what cօռspιʀαcιҽs have happened under cloak of darkness, etc.)

God is just. To suggest otherwise would be blasphemy.

Indeed, this was what I was hoping would be the answer. But I was wondering is there any docuмent, statement, etc. giving any sort of official Church view on this matter.

There might not be anything specifically about antipopes, but there is official teaching on the subject of invincible ignorance.  Following Matthew's reasoning above, it is clear that this applies.

From the Wikipedia article on "invincible ignorance" 

The term "invincible ignorance" has its roots in Catholic theology, where — as the opposite of the term vincible ignorance — invincible ignorance - ignorance that is not the fault of the ignorant person. From the Latin meaning unable to be overcome.. The first Pope to use the term officially seems to have been Pope Pius IX in the allocutionSingulari Quadam (9 December 1854) and the encyclicals Singulari Quidem (17 March 1856) and Quanto Conficiamur Moerore (10 August 1863). The term, however, is far older than that. Aquinas, for instance, uses it in his Summa Theologica (written 1265–1274),^[1] and discussion of the concept can be found as far back as Origen (3rd century).

Material schism is not eo ipso a sin.

That’s why I’d rather be a Sedevacantist any day of the week than someone who nominally acknowledges the Pope but practically rejects his papacy.

That's correct. The Dominicans gave a conference early this month, and Father Marie Laurence mentioned that Our Lord was communicating with St. Vincent and answering his prayers during the time in which St. Vincent was believing the anti-pope was the true pope, yet Our Lord did not mention to St. Vincent that he was following an anti-pope.

Thanks for this.  It definitely gives me food for thought.

That's correct. The Dominicans gave a conference early this month, and Father Marie Laurence mentioned that Our Lord was communicating with St. Vincent and answering his prayers during the time in which St. Vincent was believing the anti-pope was the true pope, yet Our Lord did not mention to St. Vincent that he was following an anti-pope.

The circuмstances were entirely different. This was the time of the Great Western Schism when three men had been elected pope by the Cardinals, and all three men claimed to be pope. It was nearly impossible to know for sure who the true pope was.  This was a case in which all three popes were truly "doubtful".  
In our day there is one person elected by the Cardinals who claims to be pope, and unfortunately it is Francis.  Ratzinger quit, his resignation was accepted by the Church, and he no longer claims to be the pope.  There is only one person who claims to be Pope.  There is no comparison between our time and the Great Western Schism.