I bought the metal hose, the longest one available. I think it was 79" or something like that. No cleaning woes. The length is good for reaching all corners of the shower. We have the Relaxa head, no complaints. I'd get it again. We've had it for a couple of years, no drips out of the hose fittings. The hose stays flat against the wall, too, no twisting and bowing out into the shower space. Mongo

I have a low-end Hansgrohe Chroma 100 Vario with a wall bar and I am very happy with it. It was really cheap and is well made and nice to use. Lately I have noticed that many hotels I happen to visit installed that very same unit. My parents put high-end Hansgrohe in their house and their handhelds perform in the same way (but their look is way nicer and they use heavier materials.) If budget allows, you might want to have a look at showerhead/handheld combos to get the best of both worlds (example in the link below.) Here is a link that might be useful: Hansgrohe Showerpipe

The handheld that you linked to (Hansgrohe 06364005) can be used with any shower supply valve. It appears that it comes with the wall outlet, the bar, the slide bracket, the hose, and the head itself. You can indeed use that with any valve. The valve body that you linked to (Delta R10000-UNBX MultiChoice Universal Tub and Shower Valve Body) has the typical hot and cold water inlets, plus an outlet to fill the tub and an outlet to supply the shower head. The valve trim kit that you linked to has the trim for the valve, which you'll need. It has a shower arm, the escutcheon plate for the shower arm, and shower head, you won't need any of those. But it does not have a tub filler spout, but from your post it doesn't seem you'll want one. For use without a tub filler spout, you'll want a threaded 1/2" cap to cap the tub outlet port on the bottom of the valve. If you're using copper and soldering things together, you'll want a 1/2" female threaded-to-soldered copper fitting to thread in to the valve's top outlet port, something like this: Your run of 1/2" copper tubing will be soldered to that fitting and run up the wall where it'll be soldered to a 90-degree drop ear fitting like this: Into the threaded end of the drop ear you'll thread a threaded nipple. The length of the nipple depends on the thickness of your wall, how deep in the wall the drop ear fitting is set, etc. I have a collection of nipples from 1" to 3" long. If you buy extra lengths, you can return the ones you don't use. So one end of the nipple gets threaded into the drop ear fitting inside the wall, the other threaded end should stick out into the shower about a half an inch (or as needed) and your handheld's wall outlet gets threaded on to that.

"What is it about this myth that causes it to refuse to die?" The fact that it's not a myth. They do manufacture products exclusively for the big box stores that look just like the better versions. Sometimes they even have the same name. For example, Delta's kitchen faucet Foundations, #10900LF is listed on Delta's site as "only at Lowe's". They also have a Foundations faucet #B1310LF that does not have that designation. As millworkman said, if the item numbers are the same, then they are the same faucet. But just having the same name does not mean they are the same or manufactured to the same quality. I think you will be better served with a slide bar. Your grandson will not be 5 any longer next year, or any year after that.