Essentially, in my humble opinion, the argument that one can have both the cake and eat it, i.e. Jesus was separate from God and still be an integral part of God, completely rests on semantics (as is already visible with the "Elohim is plural" argument). Every time you ask somebody to explain it further, there is a lot of hand-waving and an eventual citing of lines and phrases in the hope the questioner will go away.
At least I find it difficult to marry the fact that Jesus says "God, why have you forsaken me?" with the idea that he IS God too at the same time. If Jesus is so distinctly different from the remainder of God that a) the remainder can "forsake" him and b) he does not know the reason for that forsaking, the two entities only have a tenuous connection at best.
So yes, I find Islam's criticism of Christianity's practice to worship Jesus, Mary and the saints to be polytheism not exactly far off the mark. You also barely ever hear any emphasis on the supposed monotheistic aspect of Christianity, for good reason. Whereas Judaism and Islam really stress that point.
rumborak