Apple's 64 bit strategy was in place by 2007 (It would hardly make sense to omit some 64 bit Cocoa APIs in MacOS X 10.5, only to bring them back later, would it?). Forstall left in 2012. Your alternate history timeline may need some work ;-)
I don’t think so. Was there a projected date for completion of the migration? We’ve only got pervasive 64-bit in the last 6 years so doubtful it would have been before then.
As far as I recall, userland APIs had all their 64-bit support in place in 10.5 (2007). Kernel 64-bit support came in 10.6 (2009). Which 64-bit features are you thinking of in the last 6 years?
To clarify, I'm referring to macOS releases. On iOS, 64-bit support obviously came within the last 6 years, but Carbon was never in the picture on iOS anyway.