Shower mixer catastrophic failure. Help.

Hi all,
I have had a complete failure of my shower mixer and am left with a couple of questions.

The mixer failed and I couldn't stop the water pissing out all over the shower so I turned off the cold feed with the lever tap in the hot press but if there's a lever tap to cut off the hot water I couldn't find it.
I tracked the hot water feed to the shower. I goes from the hot press up into the attic, across the attic to the en-suite and down the en-suite wall to the mixer. I cut the qual-pex pipe and fitted a lever tap in the attic so I could turn the water back on in the house but cut all water to the shower.

So my questions are;
Is it ok to have a tap on the hot water feed to the shower? I assume it is but maybe there's a reason there wasn't one on it.

2. I assume the pipes that come down the en-suite wall would be copper? Or could they be qual-pex too? I'm asking this because I'm looking at a replacement shower with a drench shower that needs a fixing directly over the mixer but I'm worried that I could drill into the shower feed if I try fixing here as the pipes come straight down as far as I can tell.

3. Finally, would a stud detector pick up pipes? or should I just abandon the idea of a drench shower?

Thanks all.