Is it really technically impossible to access the Quartz APIs or write Mac applications using C?
Well it is possible, for example iTunes is non Objective-C Carbon (API) app AFAIK. Problem is that Carbon (pure C interface) is considered as deprecated by Apple and may disappear from future releases of OSXes at all, most 98% of applications are Objective-C Cocoa anyway. Moreover most of the functionality introduced in 10.5 or 10.4 went just into Cocoa, and never was backported to Carbon. So it isn't matter that it isn't possible, but it is IMHO more reasonable to do it in Objective-C.
Best regards,