I've had this problem a few times too. There is one thing to try before doing anything that will further round out the allen key hole.
This trick comes from realizing that the grub screw isn't actually 'seized' (ie corroded in, where corrosion products have greater volume than the original metal, so the threads are seriously locked in.) Nor is it spalled (metal graunched off the threads, making a jam.)
It's more likely just very tightly screwed in, ie there's a force where the grub screw tip presses into the shaft, and an equal force spread out over one side of the threads in the knob. That's ONE side of the thread, not both. Hence if the shaft could be magically removed (so no force anymore) the grub screw would turn freely, even with a rounded out allen key hole.
But the thing is, you don't have to remove the shaft _entirely_. You just have to make it not quite so large, right where the grub screw touches it.
And that is easy!
For this you need a small pin punch (flat end), that fits in the grub screw hole. Best if it is a close fit, so the end rests on the top of the grub screw. Then arrange some side support for the knob, that can take a strong force without damaging the plastic. Scrap piece of wood, use a hole saw to cut a hole that pretty closely fits the circumference of the knob. Cut one side off so you have access to the grub screw hole. Get this support resting against something solid, or in a vice, take up shaft sideways slack so the bearing sleeve doesn't take the impact...
Now whack the grub screw downwards into the hole, with a sharp impact from a small hammer on the pin punch.
The aim is to drive the grubscrew end slightly into the shaft, deforming the shaft metal (typically brass) just a little bit. This relieves the screw pressure. It will also break any adhesion between the grubscrew and the knob threads.
It's surprising how small an impact will work.
Now the grub screw will be loose, and easily backed off enough to remove the knob.
If you still have trouble turning it enough to remove the damaged grub screw entirely, once the knob is off you can turn the grub screw *inwards* more easily, till it falls out the central hole.