The injury that you have shown of the neck area does not look very severe. If you apply an antibacterial ointment, or squirt a poultry spray over, for the next few days, I think that would suffice as far as treatment.
If the males are to the point where they are physically pulling feathers from her head region, it is likely they might also be pecking the eye which would presumably create an inflammatory reaction. As you are already working on, the best treatment for the eye problem would be reducing the amount of mating, but for now, simply continue on flushing the eye with the gel you have.
The process I use for the application of ocular drops is to grab the duck in an enclosed area, and position the duck so its body is held between my thighs/knees. To do this, firm pressure must be applied, but not too hard where you might restrict respiratory flow. You can use your less dominant hand to hold the duck's head, slightly tilt it sidewards, and use your dominant hand and apply a drop or two to the eye. If the eye beings to look worse you may need to use a product such as Terramycin with oxytetracycline.
Here is a picture online I found of the positioning I use.
View attachment 2620363