Darn, that's what I thought. Problem is the flat washer side connector fits the valve, the other end with the tapered washer doesn't (too big, won't screw onto the valve). I wonder if have to get an adapter or something.

Starving of other fixtures is entirely dependent upon how much water is exiting each faucet. The size of the line to your manifold will have a much smaller effect than the number of fixtures you are using at the same time. That said - follow code as it is there for a reason.

I briefly tried a 5/8" deep socket (also note that a 15mm was too small and a 16mm too big). Was pretty sure I was just going to tear off the corners of the nut with that. It was weird because nothing felt like a really snug fit that wasn't going to rock (and then put all the force at the corners). A 5/8" wrench had a decent amount of wiggle, and even the crescent tightened down to fit the nut snugly still had some give. What other ideas are there for breaking this loose? I remember changing the anode rod on my water heater years ago and feeling kinda dumb when I finally got the right tool and it came loose easily. But I don't think this is that simple. Use a torch? Would using a punch and tapping on top to jar the threads or something maybe help? Other ideas?

Is it such a big deal for a contractor to work with a faucet that does not come with water supply lines? I got a waterfall-type Pfister faucet that comes with metal connector lines (1/2 inch connector?) , but my contractor says this wouldn't fit, and I should get ones with supply lines included. I asked him if he can just get the supply lines at the store and he says it will be hard to find them since many are out of stock right now. I know some stocks are low in stores these days, but Isn't it unlikely these type of generic parts go out of stock? Are Pfister products complicated to work with or something?