Agnostics by definition are those who have not yet decided that the evidence is sufficient to believe in God or Jesus or any form of Deity - you are thinking of Gnostics, there are christian gnostics who will happily state Jesus died on the cross and was resurrected, he is "The Son of God". The notion of a "substitute" is not something I've spent any time on, nor have any interest in - I've yet to see anything "substantial" on the matter so I'll let others address it if they feel it worth the time tbh.
Muslims do not accept that Jesus died on the cross, rose from the dead, ascended into heaven and is the second person of the Trinity. They consider him to be a prophet only.
Yes, you are right, neither do Jews. But, perhaps that is the subject of a new topic, as this one is concerned with whether the being referred to as Allah is the same being as referred to as God/Yahweh in the Bible.
But, in passing, both Jews and Christians also refer to God as Abba/Father. Muslims never call Allah their father.