I think this is a very good question, especially for people who are discussing theology in such a public way as we are (on a message board). I hope more people give their opinion on this matter.
All these heretical beliefs (some are clearer than others), is this something that God will judge us by on the day of judgment???
I think part of it has to do with how you came by the belief, and also how much you publicize it and how much influence you have. For instance, if you were reading a known heretic and picked up a heresy, I would think there'd be less leeway than if you were reading a recognized saint and picked up a heretical thought. Also, as the Scripture says, those who presume to teach "shall receive a stricter judgment." (James 3:1) I've wondered why someone like Origen got condemned, while someone like Gregory of Nyssa remained a saint. I don't know the full answer to that, but I think at least part of it probably has to do with how much influence each had. That is to say, Origenism caused major disturbances in the early Church, while we don't find people getting into arguments and fights over what St. Gregory of Nyssa taught. Thus, because Origen's ideas had more influence and a greater effect, Origen was also held to a higher standard and received a stricter judgment. Meanwhile, anything unseemly in Gregory of Nyssa went mostly under the radar, thus the Fathers "covered his nakedness" any time an unorthodox belief he held to came up.
Can you reach Theosis harboring a heretical belief???
Good question, and I honestly don't claim to know for sure. There is an idea in some of the Fathers that (true) knowledge must proceed (true) faith. On the other hand, the Church has obviously recognized the sanctity of people like Gregory of Nyssa, Ireneaus, etc., who may have made mistakes along the lines of what you are asking about.