ca_flyer,
There is a difference between acting as PIC (91.3) and logging PIC (61.51 (e) (1) (i) )
In both the Catalina checkout and the IMC lesson, the instructor is acting as PIC, and in the latter performing the role as required by 61.57 (c).
The student may log PIC purely because there is a FAR which lets him do so. He is not acting as PIC in either case. If the plane runs out of fuel, in both cases the instructor will be the one explaining himself to the Feds, not the student (even if fully rated on the plane, and manipulating the controls, and therefore logging PIC)
Cheers