That amalgamation stuff seems very very wrong to me. Not that I pretend to understand the Chalcedonian definition, but amalgamation sure doesn't come to my mind as a word to use about the two natures in one person. YEAH, I can appreciate the avoidance of "dividing the person" but that sure sounds like confusing (Literally, that seems to be what 'amalgamate' means - to make a tertium quid by smooshing (a term much loved of the Fathers, I'm sure) up two other things. Wrong-O! (IMHO, of course.)
But Again, I only was referring to, like, the first chapter or so of Mere Christianity not to the entire Lewis corpus, be it never so amalgamated.
He also has a tendency to give everyone a little credit, and to appear perfectly neutral. That is just too "attenuated" for my Orthodox mind (and I imagine to any Catholic as well), because God does not like lukewarm. God is fire and fire is hot! Ecumenism is lukewarm. We are not some universal "church," but Church Militant.
I think Lewis was trying to find a "Unified Theory" of Christianity, sort of like the attempt at having an "amalgamated", unitary Gospel (something that was tried in the early life of the Church and failed).
The Chalcedonian 'definition' is based on the belief that Christ is a perfect God and a perfect Man, to which I add, without undue influence of either on the other.
For if Christ is a perfect Man because He was getting "help" from his divine nature, then what effort was it for him to resist sin and obey God's law perfectly? And how can we imitate him then?
That in an of itself tells us that amalgamation would prevent the Church from formulating Christ as perfect God and perfect man.