Under FAA rules you can log PIC any time you are rated in category and class, and sole manipulator of the flight controls.
But just because you are logging the time, doesn't make you the actual PIC. During IR instruction, the instructor has to be the actual PIC, since you're IMC.
For the safety pilot example, you must agree before the flight that the safety pilot is the actual PIC, while the pilot under the hood (You must be VMC for this) can log the PIC time as the sole manipulator.
By the letter of the rules, you could also have an instructor in the back seat giving instruction and all three logging PIC time.