So you have the option of running the baseband on the application processor to save costs if need be, among other reasons.
You need a hypervisor for that. Qnx brought that later.
You don't. Symbian EKA2 platforms (eg. S60 3rd edition and later) used to run baseband as userspace threads on same core as user applications. I would not be that surprised if that was main enabler for Nokia E51/52, ie. full-featured smartphone in executive-phone form factor (small and in the first place thin candy-bar).