A half quart every 5,000 miles is totally normal oil consumption, especially when you consider Kia says up to a quart every 1,000 miles is "normal" with their recent oil burning recalls! That's not supposed to be normal. That's not really blowby that's coming out of the oil filler cap. The pistons and crankshaft are whirling around inside the block, essentially pumping air around. There's always air in the crankcase, and they used to vent directly to the atmosphere until the EPA put a stop to that due to hydrocarbon pollution long ago.

With the cap off, the air that would normally be pumped around inside the engine is coming out through that opening instead of being pushed through the PCV system. The PCV valve is basically a check valve and lets air flow forward, through the valve and into the intake, but not backwards, into the crankcase from the intake. The engine can remove the crankcase air with vacuum pressure, but air can't flow back. Positive pressure on the intake side closes the valve. You're bypassing the check valve when you remove the oil cap.  

Toyota 1AZ-FE are known to strip the head bolt threads - but I do not think that's the case,

The compression rings are probably just worn out, I'd replace the PVC valve anyway and hope for the best.

After replacing the PCV, You can do a dry/wet compression test to see how bad it is.