757 HYD
Q1. The PTU operates when the L HYD has fluid but no power. The fluid might be the complete L System or just the Stack Pipe reserve. The PTU is driven by the R System but there must be fluid available for it to work (and a serviceable R System).
Q2. Yes, if the leak is below the Stack Pipe, there will be no fluid left and thus no PTU. That is why the QRH says ALT Gear/Flaps etc MAY be required. You might not know where the leak is and thus not know until you select Flap 1 whether or not anything is going to happen.
Q10. Have a look at QRH Unsched Stab Trim. What do you do (without lingering - hopefully)? Think what those two switches are and therein lies the clue. It helps to work these things out yourself because that way you get a much better understanding of the whole aircraft.
It is some years since I last had a complete L HYD failure and I am sure there have been many mods since, but my experience was a total loss of fluid from below the stack pipe. That caused a Status PTU msg (indicating that it was NOT going to work) and zero L HYD pressure. EICAS got to page 3!