Loop sliding on prime 4

I know that I can use shift + the loop knob to move the loop by its entire length, or the in/out buttons with the jog wheel to move the start or the end. Is there a way to move both the start and the end at the same time with the jog wheel, to tweak the part of the song being looped whilst continuing to play in sync?

No.

But you can see if anyone has already made a feature request for it, in the appropriate section.