I don't think our VQ35DE engines are known sludgers, and they don't have any piston ring problems as far as I'm aware. The only potential defect that can contribute to oil consumption is the passenger side valve cover baffles -
read here. But you're only consuming about a quart every 5k to 7.5k - this is well within acceptable limits and is not excessive by any means, so I wouldn't worry about replacing or cleaning the valve cover.
M1 HM is an excellent oil, and your interval of somewhere between 5k and 7.5k is perfect. Our engines are known to shear oil rather quickly, so I wouldn't go past 7.5k even with a top-tier oil such as Mobil 1. Why do you choose 10W-30 oil rather than 5W-30, though? The 5W-30 will be thinner at startup, which is desirable to reduce start-up wear, while remaining the same viscosity at operating temperature. What oil filter do you use?
I would suggest removing the lower intake manifold while doing the power valve screws to replace the PCV valve. You'll need these parts to do the power valve screws and PCV valve:
The cost for all of the gaskets from the dealer is rather excessive (somewhere around $55 to $70), so you may want to consider
an aftermarket gasket set. This one doesn't include the throttle body gasket (it's around $5 from the dealer, just get it while you're getting the PCV valve), and the bottom two gaskets in the picture you wouldn't use in this case, but ultimately it's still cheaper than buying all the gaskets from the dealer. I would absolutely buy an OEM PCV valve since it's in a horrible spot, and it'd be irritating to re-do the job just because of some defective aftermarket PCV valve. If you choose to purchase the gasket set from Advance Auto Parts, order online and use discount code
VISA, then pick-up in store to knock the price down to $24.59 before tax.
As for the P0430 code, your passenger's side cat is below efficiency specifications and should be replaced. Since you replaced the downstream O2 sensors
twice, I'd say that rules them out as a possible cause of the P0430 code.
How many miles do you have on your Pathfinder?