I would say that the prophet Muhammad is indeed in Heaven.
I am a sincere and dedicated Christian who also upholds the doctrine of universal salvation. I cannot fully justify this belief, but it is the secret hope of every sane and decent person. I also realize that there are logical difficulties, but every religious belief has its inconsistencies.
Now back to Muhammad, the last and greatest of the Islamic prophets. While Jesus Christ is unique among the founders of the world religions, the others were also commissioned by the One God to spread the message of divine commandments to all parts of the world. Gautama, Confucius, Muhammad, and the others were all legitimate prophets, even though Jesus was the greatest of them all, as being the incarnate Son of God and the resurrected Christ.
The Qur'an honors Jesus son of Mary as a great prophet and attests to his virgin birth and sinless life. The very least that we Christians can do is to appreciate that fact.