Does he mean that there is little bit of fluid left which is considered or sensed as zero but that little bit is enough for the PTU operation
I think you may be confusing pressure and quantity.
The PTU is using hydr Sys B fluid (reservoir) and syst A PRESSURE if sys B PRESSURE is too low(under 2350psi).
If sys B quantity is lost,The PTU will not work and you will follow loss of sys B (quantity) NNc which tells you arm the stby hydr.(alt flaps to arm).
The stby hydr has its own electric motor and hydr pump for the LE flaps/slats,this will allow their extention.
It is safe to take off with LE down but less to land with the LE up
Pls note: As far as I think the Standby Hyd pump cant retract the LE devices only if it has been extended by the ALTN FLAP switch.
Correct and your QRH says so.
3. Now if I am at cruise and I have lost EDP in Sys B and I come in for landing. My flaps are still up, now when I want to extend my flaps to flaps 1 the PTU here wont help me in extending the LE devices ( since the condition "flaps less than 15 but not up" is not met.... My flaps are up!!!!)
Correct,you will use the standby hydraulic fluid and pressure to extend your LE flaps and the stanby electrical motor to get your TE flaps down.
Try to think the PTU as an automatic help to get your LE flaps up and down if sys B PRESSURE is a bit low.