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https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/care-knockout-roses.htm

Prune out the dead canes and wait and see. They may just need a bit more time.

https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/prune-knockout-roses.htm

I would consider placing some plant cages around the small plants. Even a tomato cage would protect them until they have grown a bit larger.

You can also make a little cage with chicken wire; easily found at most hardware stores.

https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/care-knockout-roses.htm

The holes in the leaves are likely due to little beetles that have long snouts. They poke into the leaves and suck out the juices. I would recommend buying some Bayer Tree and Shrub granules. Apply the granules per the label instructions for ornamentals/shrubs. The granules have some food in them to help strengthen the rosebushes and also a systemic that will kill the beetles and some other insects or worms that would suck the juices out of the leaves. You could also use a systemic insecticidal spray if you wish to accomplish relief from the problem.

Treatment with an insecticidal soap or Neem Oil is recommended.
Try watering around the base of the rose and not down into the crown.
These links have more information.

https://www.gardeningknowhow.com/plant-problems/pests/insects/skeletonized-plant-leaves.htm
https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/knockout-rose-diseases.htm

I trim them off because it looks better. Knock-Outs aren't known for producing colorful rose hips if any form at all. Removing spent blooms also gives me a chance to look over the bush for rose slugs, mites, Japanese beetles and other, less common, pests. But it is not mandatory and doesn't affect future bloom rate one way or the other.