The pharisees condemned Christ because they knew he was referring to Daniel 7:13-14 when he said they would see him coming in the clouds. In other words, they condemned him for claiming to be the fulfillment of that prophecy. Has nothing to do with your imaginary modalism thing
Do you realize that the Son cannot be a mere mode of the Father and simultaneously be right this moment in Heaven also sitting at the right hand of the Father at the same time? Do you realize the obvious implication here in the Session of Christ, a key facet to the Gospel of Jesus, that there is one God in three persons which is the verity of the trinity doctrine?
Not that you need to answer these questions here, but ponder on them a little bit. So it is that the Christian religion is held up by two pillars; the trinity doctrine and the Gospel of Jesus, which this religion is certainty of salvation. The Oneness modalist cult has to deny both these pillars and therefore is firmly outside of the religion because of the pernicion of their heresy and the consequences of that are certainty of damnation. Due to the length of eternity it's worth sparing a little time now to think about.