I would go ahead and repeat the Ivomec as scheduled. I would not expect it to do anything for limping since it's an antiparasitic used to treat lice/mites and certain worms. But it's always best to finish a course of treatment once started. You won't have to worry about lice or mites for a while though!!
Try the Vitamin B2. You can also give a weak aspirin solution to see if that helps as well. If it's sprain or injury that may ease it a little. 81mg Aspirin (1 low dose Bayer) to 1 quart of water. Sweeten with a little honey to make it more palatable, but I would also make plain water available too, some birds just will not drink Aspirin water and you don't want her to get dehydrated.
Marek's can be frustrating, the signs and symptoms seem to be all over the place, but I agree, the way she is holding it up, it looks like it's painful to her. Can she "fold" it up? I wonder if she has a tendon slipped out of place - feel her normal leg at the back of the hock and compare it to the hurt one - you should be able to feel if a tendon has slipped out.