I don't think the "Islam is a christian heresy" is available in English. We, who know arabic, are not doing a good job in translating books that might be of great value to mankind.
Your conclusion is right and this opinion is maintained by many scholars. Many believe that Muhamed started out as an Ibonian " christian", who was hoping to be able to unit the many "christian" sects under one religious leadership. It seems that lust for power changed the whole goal. However, in the verses of Quran, specially the ones at the beginning, are very peaceful towards the "christians".
The Ibonian heresy is discussed in some details in Origen's books and I think St.Ireneous also discusses it in his books against heresies. I have to confirm this. I am also not sure whether a complete copy of the "Bible of the Hebrew" still exists. It is supposed to be taken from St.Matthew's Gospel, but it is like an amplified version, and also in many verses falsefied.
The heresy maintains:
1- The Lord Jesus Christ is not the son of God, he is a superior being , superior to all prophets. It is close to the Arian heresy and the heresies of Paul of Samasota, and in some instances it turns Nestorian. The maintain the the Messiah left Jesus before the Cross.
In Islam, there is a special place for Issa (Arabic for Jesus) that no one can miss, although the muslim scholar tried their best to dodge it and downplay his position. Some verses are striking, like "Issa (Jesus) the Spirit of God and His Word". Muslims do not know what His Word means.
2- They deny the Trinity
Deny the Trinity, of course. One difference between Christianity (the Orthodox faith, of course) and Islam in their view of God. Muslims maintain that God's attributes can be "admitted" to his person, meaning, they are not eternal. Although they do not confess that clearly, their theology just confirm that.
3- They maintain all Jewish rites as in the OT. This is very important, as Islam also has taken many of it. They don't eat Pork, like muslims, no wine, have to wash before prayers, have a degrading look towards women, have divorce, ...
It is interesting.