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I can only answer to the rosebushes. Please see this article link as it speaks to your question. Knock Outs are any different than any other shrub rose when it comes to planting them. Link: https://www.gardeningknowhow.com/ornamental/flowers/roses/spacing-roses.htm

I suspect that they are getting too much nitrogen. High nitrogen fertilizers, some actually called Bloom Busters, cause the production of more growth of foliage and limited to no blooms. Here is a link to an article I wrote on the subject for you: https://www.gardeningknowhow.com/ornamental/flowers/roses/no-blooms-on-roses.htm Giving them each a cup of Kelp meal and some Super thrive mixed into some water that they are watered with will help get them going. Use an organic or organic based rose fertilizer that is well balanced on the NPK will help alot too.

The quickest way to stop black spot in its tracks is to spray the rosebushes well with a product called Mancozeb. You can get it at Amazon.com. Part of how it works is to leave a light yellowish powdery residue on the foliage. But it does work very very well. If it is a severe case of black spot it may take two to three sprayings 7 to 10 days apart. Keep in mind that even when the black spot fungus is killed it will not just disappear from the foliage. It leaves its mark! However the new foliage coming on should be free of the black spot if the fungus has been killed. Take a look at this article too: https://www.gardeningknowhow.com/ornamental/flowers/roses/disease-resistant-rose.htm

The Japanese beetles like pretty much all roses. So a systemic insecticide that gets at both adults and grubs is recommended.

Any rosebush could be susceptible to the Rose Rosette virus. However, if you remove and destroy the roses, remove all root matter that you can from the soils, then spray the entire area and any other rosebushes with a miticide, planting other rosebushes is still a good option. I would first recommend taking a look at the Home Run series of rosebushes by Weeks Roses as they have similar growth and self cleaning habits. The original Home Run and Watercolors Home Run as especially nice. There are also many beautiful English rosebushes at David Austin Roses.

It sounds like your knock out rosebushes were grafted rosebushes. At some point the hardy root stock that the top part was grafter onto got over zealous and decided to send out shoots to grow. It is best to prune out these shoots all the way down to the ground and maybe even a couple inches below ground. Then watch for the shoots coming back, usually the foliage and growth characteristics will be noticeably different, prune them out as they are discovered. Otherwise they will sap strength from the actual Knock Out rosebushes and could lead to their death, preceded by poor performance.