If the lanes themselves don't move through space along with the galaxy they're in, then the whole system is a wash. But if they do (and from your description, it sounds like they do), then under the parameters you've laid out, you may not actually have the issues you're imagining you might have.
Remember that a galaxy doesn't just rotate, but moves through the universe, as well. As you describe them, your warp lanes are fixed with regards to the galactic center. If this is so then that means they are traveling along with the galaxy at large; if not, then they'll be sitting alone in deep space as the rest of the galaxy quickly leaves them behind. So, if they're traveling along with the galaxy, why can they not also orbit the galaxy just like every other occupant?
Now, at first glance, that may not seem to solve your problem entirely, since star systems all orbit a galaxy at different rates. But, since you mention leeway of up to 20%, you probably don't have to worry about this at all, since it would take many thousands of years of relative drift between systems for a warp lane to "stretch" past this limit. Alternatively (or additionally), you could set up multiple endpoints in advance to supercede older ones as planetary systems shift in distance to each other, but that would be planning way in the future.
Also, as a much more minor concern, keep in mind that a solar system's orbital plane doesn't necessarily line up with it's parent galaxy's plane. Our own solar system's orbital plane is at roughly a 60 degree angle to the Milky Way's orbital plane. As you design your warp lane systems, angles more favorable or more extreme to each other may have some impact on the ideal/optimal endpoints for your lanes on a case by case basis. (E.g., one lane might be able to line up essentially connecting one inner planet to another inner planet, whereas in another case, a lane is better suited exiting on the outskirts of the system.) But even then, the angle between systems would need to be quite close to 90 degrees for this to be an issue.
Update in response to comment concerning "twisting and turning" of the warp lane:
Such a twisting motion doesn't need to occur. 801
The factors affecting an object's position don't equally influence its orientation. To use a very simplistic analogy: if your warp lanes are like "tubes" through space, they don't need to be tied down at their endpoints, they just need to reside (orbit) at their desired positions.
Theoretical warp lane constructs like you're describing would have, by necessity, some measure of "tension" to them. So, continuing with my extremely simplistic analogy: imagine a hose dipped into a basin of water (on the surface of the Earth). If you let a hose sink down partway into the water, and then you start rotating the basin, then sure, that hose will be subject to the influence of turbulence you've just created, but the dominant forces affecting it's position will be hydrostatics and gravity, same as if the basin were at rest. If we performed this experiment at the scales you're concerned with, the effects would comparatively be even more negligible.
It's not a perfect analogy, of course, but this is a lot like how the dominant gravitational forces affecting the endpoints of your warp lanes would exert themselves. The endpoints aren't relatively "glued" to their locations, they're in orbit at their locations. The rotational influence of nearby bodies (i.e., the "turbulance") won't be enough to disrupt the stability of your warp lane so that they'd start to twist in disruptive ways. Rather, it's the overall gravitational influence of the nearby objects (i.e., the "hydrostatic equilibrium") interacting with the mass of the lane itself that exerts the most influence.
Granted, these descriptions are relative depending on the perspective of an observer. But the overall notion that stable warp lanes don't necessarily exhibit catastrophically destructive twisting effects is maintained, regardless. The simple reason for this is, if they were so easily affected by such forces, they wouldn't be stable enough to exist in the first place. 801