Two cues I don't follow. Let me use your journal for a little preaching.
Having the elbows "back" results in the opposite of what you want. Pushes the chest down and forward instead of up and back. Not to mention putting you in external rotation and causing neck hyper-extension in order not to have your head down...which basically messes you up like a chain all the way down.
As far as arch and arch is an arch. Normally if someone is walking around with an exessive lower back curve, that's a bad thing. Yet most people say to "arch your back" meaning to "hyper-arch" your back. It's not better to bring yourself into excessive pelvic anterior tilt than it is to tend toward posterior tilt. Hell, this is the kind of stuff that leads to the squat being called a "quad dominant" thing instead of a hip dominant thing. EMG says it's all quads but Mr. EMG doesn't know thing one about proper balance.
Hell, the notion that any lower body multi-joint movement should be "dominated" by one muscle group is ludicrous in and of itself. Come on folks, does that sound like how the body works? The thing I said about 'ass dominant' was even a joke to make a point. Extensions are quad dominant
. If you're squats are only in your quads (and hip flexors) then you have mobility problems.
So which comes first....improper movement patterns that basically cause the gluteals and hams to be unable to do their job causing the hip flexors to get tighter and tigher...well, basically turning it into a thing where the quads are doing it all.
If you exxagerate the arch you prestretch the back, hip extensors, the glutes and hams, tighten up the flexors all of which results in more lower back strain, not less, the inabiltiy of the glutes and hips to fire probperly, etc. and so on. Hell, it can even end up screwing with your shoulders too.
Just remember, pre-stretching a muscle does not help a muscle get stronger...it inhibits it.