You're making an assumption here: that less than optimal mechanics are important causes of injury and pure time spent practicing is not.
I don't go about blindly guessing at things. I also did not refer to "optimal mechanics". I am going by:
- personal experience as an adult student, both in regards to problems first time round which came close to creating permanent injury, and in regards to alternate approaches and the good things that created (as expected)
- having worked with / helped students in that situation and watching things improve and change. Often that led to a leg up to proper teaching / better teaching by another teacher. Again it has been observation.
- what I have been told privately, in depth, by a few teachers whose opinion I have reason to trust. Again a lot of factual things.
... there is also a point at which too much time causes injury regardless of how good the technique is.
Undoubtedly doing anything to the point of exhaustion will cause muscles to seize up, the mind to lose alertness and therefore bad injurious movements etc. But if spending too much time can cause injury, that does not mean that poor movement and no guidance does not.
Recreational students don't tend to get injured, at least we never hear about it on teacher forums.
You don't in fact know whether "recreational" students get injured. You also cannot base yourself on the bits and pieces you can glean on a teacher forum. There is very little interest in this kind of student, so it's not discussed much. There is also misteaching of this kind of student, and thus wrong teacher attitude can lead to problems.
We do hear constant complaints about them not practicing enough though! (and paying late, and missing lessons, but not getting hurt).
We also hear constant complaints about child students not practicing enough. Do you really think that if a teacher has a student who got injured under his watch, that he's going to advertise it?
Serious students do get hurt, as a large number of studies show.
Studies will study these students, in particular types of institutions.
Students at the level of competing for those few conservatory spots didn't get to that level without excellent instruction. They are the best taught of any piano students.
Do what I did. Start talking to people in private. Hear their stories. You'll get a few surprises. You can get instructions that will make you excel in the type of thing you get tested on, while not getting the kind of thing that prevents injury.
(when you go to bed tonight I'll still be up - practicing YOUR part!)
I quite doubt that. I went to bed at 3:30 a.m. last night, having found some quality practice time. You were up at 4:00 a.m. practicing my part?

I will also say that I reach things in
a quarter of the time that I did in the past, because I have learned how to work effectively.
.... the original Hanon topic which we long left behind.
You may have left it behind, but I haven't left it behind. In fact, I've tried to bring it back repeatedly. I have stressed the HOW - the details are in my previous post somewhere back there.