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I am sorry about your plants. The soil may still have remnants of the chemicals used for some time but after about a year or so without the use of any chemicals, it should be ok. You can try flushing the area periodically with water to help speed the process along but until then you can always put your plants in containers or a raised bed instead.

This sounds more like something either got poured onto the soils around the roses, some herbicide spray got onto the foliage or neighborhood dogs are relieving themselves on or near the rosebushes. As long as you are keeping them well watered and fed, they should not react this way. There is more going on here than immediately meets the eye I would say. Take some soils from the ground right by the affected rosebushes and some cuttings. Place them in a plastic bag and take them to your local extension or agricultural office and see if they can do some tests. It could also be that some grubs are eating away at the root system and thus destroying the means by which the rosebushes take up food and water. In such a case, I would treat the entire area with a product called Bayer Tree & Shrub Granules. This product will get rid of the grubs and protect the upper part of the rosebushes as well. plus give them some nourishment. I would make an application of the Bayer product ASAP truthfully.

The seeds could be most anything really. Some are actually just pellets of wood that do absolutely nothing. It is just a way for some not so nice folks to steal your money. Stay clear of them.

If you mean a rose mix fertilizer be it granular or foliar application, follow the label on the product for roses or ornamentals. If there is not label on the product either throw it away or if you are sure what it is, then contact the company to get application information. Rose mix soils amendments are usually just working into the garden soils at no more than a 50-50 ratio.

This sounds like a very typical grafted rosebush problem. The upper desired part of the rosebush dies over winter for some reason. Then the lower and very hardy root stock takes over. The growth is usually tall, leggy and quite vigorous. Sometimes it will bloom in 3 to 4 years but the blooms are deep red or pink, certainly not the rose you desired back when purchased. I dig them out and plant a new rosebush in the same spot using amended bagged garden soil. Prune the old rosebush way down and throw the canes into the yard waste container. Dig out the root ball getting as much of it as you possibly can. Then discard in the trash. Here is an article for you on this subject too: https://www.gardeningknowhow.com/ornamental/flowers/roses/own-root-roses-grafted-roses.htm

Perhaps the solution was too strong and you burnt the plant. That happened to me once when I was too enthusiastic in ridding my orchid of aphids/thrips. The plant may not die (my orchid is still alive years later). I suspect the damaged leaves will continue to fall off and new ones will eventually appear. Since healthy leaves feed a plant, you may prune the bush to encourage new growth and lessen stress on the plant.