I would expect the jet stream not to end abruptly, but there is likely a gradient you have to cross before reaching "still" air. When leaving the stream, airspeed will increase, because of inertia, and so will drag. This will quickly bring the plane back to its cruising speed.
I had a debate with a friend on this one the other day, but I am pretty sure something like 80% of the engine power is there to combat drag, which increases with v² (if you discount lift from that, otherwise it' obviously 100%).
After some clarifications from replies to my sibling post, the answer is "definitely more than 50%" because the lift-induced versus parasitic drag curve is at its minimum when the two are equal, but the efficiency curve is a relatively flat-bottomed bathtub shape, so the cruising speed is selected to be faster than where this point is as you get significant reductions in travel time for relatively small reductions in total efficiency when you are near the 50% point and increase airspeed.
Depends on how you define drag. If you define it as friction due to air against the plane then no, because a lot of the drag is due to sending air downward to keep the airplane up.
The distinction is that, that that part of the drag is impossible to reduce, while friction you can work to reduce.
That's just semantics; you can split the drag generated by the wings and fuselage into different components. Of course, 100% of a constant-speed plane's thrust is necessarily used to counter all the other forces acting on the airplane :)
I had a debate with a friend on this one the other day, but I am pretty sure something like 80% of the engine power is there to combat drag, which increases with v² (if you discount lift from that, otherwise it' obviously 100%).