Have you checked out any of the construction documents for the various sized FSL the Gary wrote? There are some of them in the files section here under "Antennas and Equipment" (I believe).
I think that there are others online as well. They will not answer some of your specific questions perhaps, but will give you ideas of what is known to work.
I am not sure about PVC. It depends on what kind you use. I have read that the white PVC is the best for coil forming, but it can be lossy at higher frequencies. It could influence the Q of the system. How much at MW? I haven't seen measurements.
With the recommended inductance and variable cap my 7" FSL does not tune much below 500 on the low end. I believe that some people have built multi coil FSLs to get the lower frequencies.
Not sure if I helped, but maybe gave you some ideas for further research.
I am sure that Gary or others here can add much more info for you.