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I would prune out some of the worst looking portions and then spray treat the other areas. Spray the bush as recommended on the label of your product of choice.

Here are some links to articles that should be of help to you. Knock Out roses are no different than other rose bushes when it comes to planting them. Links: https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/care-knockout-roses.htm

https://www.gardeningknowhow.com/ornamental/flowers/roses/planting-rose-bushes.htm

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

https://www.gardeningknowhow.com/ornamental/flowers/roses/location-to-grow-roses.htm

Normally, roses are pruned in spring or in fall when dormant. However, others say that is old advice and pruning during growing season allows quick healing of cuts.

You can cut shrub roses back by about half if needed. Cutting back climbers may sacrifice blooms if next year's blooms are already developing. If they are Knock Out roses, they are pretty forgiving and you can cut them back by half now and it shouldn't hurt them.

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

https://www.extension.purdue.edu/extmedia/HO/HO-4-W.pdf

https://www.gardeningknowhow.com/ornamental/flowers/roses/pruning-climbing-roses.htm

Did you mean Sevin?

I use a rotation method of getting after mites. I use Avid and Floramite. I will do a couple sprayings of Avid and then a follow up spraying of Floramite one year. Then the next year if the mites come on again, I will do two sprayings of Floramite followed up by a spraying of Avid. These miticides can be pricey so I buy smaller quantities of each on Ebay. A little of the concentrates goes a very long way. I would wash the foliage down well in the early morning or later in the evening when the temps cool down. Then spray with the miticide the next day.

I have not heard of a life span on the Knock Outs. Mine have been going strong for 6 going on 7 years. I do suggest doing a hard rejuvenating pruning on them though. Prune the back in the late fall, once they have gone dormant. Pruning the back more than any you have done before. In early Spring when they first start to leaf out, water them with some water that has both a product called Super Thrive and a Root Stimulator product in it, such as Fertilome Root Stimulator. Water them with a fresh batch of this mix the next 5 to 6 times they need watering. Also give them a good organic rose food that will build their soils home and feed them, such as this one: https://www.arbico-organics.com/product/down-to-earth-rose-and-flower-mix/Organic-Fertilizer
I also recommend giving each of them a cup of Kelp Meal mixed into the soils. They love that!

At time of the early spring pruning, yes you can prune the knock outs down to 6 to 8 inches and they will come back fine. I recommend watering them with some water that has both a product called Super Thrive and a root stimulator mixed into it at time of first spring watering and for the next 5 or 6 times the rosebushes need watering. It helps get them going and growing well. I would feed them a good organic or organic based rose food.