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Since videos were not really around before Vatican II, the Church never spoke about Catholics watching videos or online sermons from non-Catholics. I know Catholics should not watch videos from non-Catholics, even if only curious. Unless one is actively debating a topic, and they watch the video in order to refute the non-Catholic arguments. Would this also apply to videos from those who are claim to be Traditional, and are for the most part, but believe in errors which contradict the Faith and Church teachings? Would viewing a video knowing all of this be a venial sin, or a mortal sin?

There is morally no difference between what you described, and reading non-Catholic material. There may not have always been video, but there was always the equivalent, such as attending sermons by non-Catholics. 

I would encourage you to look through this book written by a Jesuit priest, explaining the governing principles behind the Roman Index of Forbidden Books. https://archive.org/details/romanindexofforb00bettiala/page/n7/mode/2up

As pornography is forbidden to us on account of the occasion of sin, so is the reading, listening, watching, etc. of non-Catholic information. The Church has historically only approved of reading dangerous books for those qualified to refute it. We can see this with Pope Leo XIII condemning Fɾҽҽmαsσɳɾყ, Pope Pius XI condemning socialism and communism, St. Alphonsus Liguori in "The History of Heresies and their Refutation", and so on. A basic Catholic catechism will explain that association with heresies is a sure way of weakening, and eventually extinguishing the Faith in souls.

One has to ask oneself; what is the utility of listening to non-Catholics? I for one have no interest in doing so, they do not have the Holy Ghost, nor the approval of the Church. Pope St. Pius X in Pascendi taught that the spiritual cause of modernism is pride and curiosity. St. Thomas Aquinas before him, also taught that these two sins are the cause of loss of Faith as well.

There is morally no difference between what you described, and reading non-Catholic material. There may not have always been video, but there was always the equivalent, such as attending sermons by non-Catholics.

I would encourage you to look through this book written by a Jesuit priest, explaining the governing principles behind the Roman Index of Forbidden Books. https://archive.org/details/romanindexofforb00bettiala/page/n7/mode/2up

As pornography is forbidden to us on account of the occasion of sin, so is the reading, listening, watching, etc. of non-Catholic information. The Church has historically only approved of reading dangerous books for those qualified to refute it. We can see this with Pope Leo XIII condemning Fɾҽҽmαsσɳɾყ, Pope Pius XI condemning socialism and communism, St. Alphonsus Liguori in "The History of Heresies and their Refutation", and so on. A basic Catholic catechism will explain that association with heresies is a sure way of weakening, and eventually extinguishing the Faith in souls.

One has to ask oneself; what is the utility of listening to non-Catholics? I for one have no interest in doing so, they do not have the Holy Ghost, nor the approval of the Church. Pope St. Pius X in Pascendi taught that the spiritual cause of modernism is pride and curiosity. St. Thomas Aquinas before him, also taught that these two sins are the cause of loss of Faith as well.

St. Thomas Aquinas spent a good portion of his life studying a non Catholic pagan named Aristotle and referenced him as much, if not more than the authors of the Gospel in his Summa. If I was driving and got lost would I need to determine if someone was a traditional Roman Catholic in order to ask them for directions?