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It sounds like you need to provide a trellis for the rosebushes you mention and tie them off to that, at least until the younger canes gain the strength they need to stand up better. They have grown up being used to the support of the older canes, once removed they need to kind of figure things out for themselves. Be sure to feed your rosebushes with a good organic rose food that builds the soils and feeds the bushes. Also give them some super phosphate to help strengthen the canes. I also recommend giving all the roses about 1/2 cup of Epsom Salts scratched into the soils and watered in well.

Sorry to have to say this.... The only black rose I have heard of turned out to be a myth. There are those that sell rose seeds on-line that are not truly rose seeds. Some are weed seeds and some are other plant seeds, some just little organic clumps that do nothing. It is upsetting that these folks get away with selling such things but they do. Here is a link to an article about growing rosebushes from seeds for you though: https://www.gardeningknowhow.com/ornamental/flowers/roses/growing-roses-from-seed.htm

Here are some links with more information.

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

https://www.gardeningknowhow.com/ornamental/flowers/roses/starting-a-rose-garden.htm

I have seen wild roses growing near walnut trees but they are generally pretty tough roses. The Juglone that black walnut trees are rich in does indeed get after many plants. Since most walnut trees these days are grafted onto black walnut root stock, we still have to deal with the juglone. I would do a test planting or two. Pick a place that is as close as you would want to plant roses to the tree. Plant one rosebush there. Then come out about 4 to 5 feet from that rosebush planting getting further from the walnut tree and plant that rosebush there. See how they do for one complete growing season and even how they come back from winter time. If they do well then you are far enough away that the juglone the walnut tree roots produce that the rosebushes can deal with it or they have no problems with it.

No it is not the fragrance, roses can have some insect problems just like other plants. Spray the rosebush down with a good insecticide such as Bayer Advanced Complete Insecticide and that should get a handle on the bug problems.

It is still likely Powdery Mildew and given time will likely move down the stems under those buds and then to the leaves. I use a product called Green Cure to spray all of my rosebushes and it works very well on powdery mildew. Here is a link to their website and places to buy it: http://greencure.net/
If you push on the white growth and there is movement or little white bugs come out of it, then the bushes need to be sprayed with an insecticide such as Bayer Advanced Complete Insecticide.