Is it normal for a plumber to block niche space with PEX like this?

That layout sounds like a classic case of a plumber taking the path of least resistance rather than following a design plan. There is no code requirement that forces PEX lines to run through the center of a pony wall. In a standard setup, those lines should be kept low or tucked to the side specifically to acommodate blocking and niches. If he had planned it out, he coud have easily run the supply lines along the floor plate and then up into the specific stud bays where the valves are located. To fix this, he'll need to re-route those lines, wich is a lot easier to do now before the cement board goes up. Using 90-degree elbows on PEX isn't technically a code violation, but it is definitely more restrictive for water flow and adds unnecessary potential leak points. The whole benefit of PEX is the ability to make long, continuous runs with **support bends**. If he's using crimp fittings or, god forbid, SharkBites inside a closed wall, you have every right to be annoyed. Those 90s are ussually used when someone doesn't want to take the time to drill better holes or use bend supports. And pipes resting directly on notched wood is a major red flag for **abrasion**. As those pipes expand and contract when water turns on and off, they rub against that rough wood. Over a few years, that can actually wear a hole through the pipe wall. He needs to use plastic insulators or suspension clamps to keep the pipe off the wood. As for splitting the tub lines instead of using the existing shower supplies, that's just pure laziness. By splitting one line to feed two high-demand fixtures, he's creating a situation where you'll likely see a massive pressure drop if both are used at once. It also suggests he didn't want to bother with the extra demo or pipe work to extend the original shower lines. You're paying for a professional install, and right now you're getting a hack job that's going to ruin your plans for those niches. I'd make him tear it out and move those lines to the bottom of the wall and secure them properly with **isopads** or plastic grommets.

Spot on. That plumber definitely took the easy way out. One thing to look for while he's re-doing it is nail plates. Since he ran those lines through the studs in a narrow pony wall, there's a huge risk of you or the tile guy driving a screw right into the PEX when installing the backer board. If he didn't even bother to use insulators, I highly doubt he put up steel strike plates. You definitely want those in place, especially around those niches, or you'll have a pinhole leak hidden behind your new tile in no time. It’s cheap insurance that a hack usually skips.

I get the frustration, but have you ruled out the possibility of structural blocking in that wall? Sometimes they run the lines through the center because there's a double plate or extra bracing lower down that they aren't allowed to drill through without compromising the pony wall's stability. Also, regarding the 90s, did you check your local water pressure? If your PSI is on the higher side, the flow restriction from a couple of elbows is basically negligible. As for the 'lazy' plumbing, is it possible the old shower lines had some corrosion or flow issues he was trying to avoid by tapping into the tub feed instead?