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Most of this depends on where you live, as warmer areas tend to prune in fall while cooler regions wait until spring. It may also be based on preference, though spring seems to be a popular time for pruning roses. This article will help you with that: https://www.gardeningknowhow.com/ornamental/flowers/roses/pruning-roses.htm

The height of the Parade mini roses can vary from plant to plant; however, I believe most of them get to be 15 to 16 inches tall. You could put two in a pot and they would fill out nicely, making for a wonderful display. I am providing a link to Poulsens website as they are the hybridizers for the Parade line of miniature roses. If you know the name of your Parade roses, such as Apollo or Fiesta, you can click on the name for more information about your particular little rosebush. I do believe most of them do get bushy, so they should fill in a 10 to 12 inch pot nicely. Just remember that you do not want them to fill in too tightly, as that limits the airflow through the bush and creates a fungus breeding ground for black spot and others. https://www.gardeningknowhow.com/ornamental/flowers/roses/keeping-a-mini-rose-houseplant.htm

I am not sure where you are but if you are in an area where the temperature is very hot right now, that can cause the blooms to open quickly and then fall apart. Many yellow blooming roses have this problem. I have had some that are in a nice opening bud stage when I leave for work and then are totally done when I get home from work on hot summer days. The feeding at 7 to 10 days is a bit heavy unless it is all foliar feedings. Even then it can be a bit much causing heavier foliage growth and less blooming with weak blooms when you do get them. I would cut back on the feeding and use a moisture meter to check the soils moisture around your roses. They may need more water than they are getting if it is hot where you are. They may also be getting too much water if it is not hot where you are. Too much water can make the soils heavy and thus lock out the oxygen that does actually move into and through the soils, this will cause lack lustre performance as well.

Stan the Roseman
Consulting Rosarian

It sounds like you have a little bit of what is called Rose Canker or die back. Sometimes it gets into the canes when pruning the roses or deadheading them if the pruners are a bit dirty. I would recommend wiping down your pruners with some Clorox or Lysol disinfectant wipes, then prune out the blackend ends and seal them with Elmers or some good white glue, but not school glue. This article should also help: https://www.gardeningknowhow.com/ornamental/flowers/roses/pruning-roses.htm

This blackening can also happen if pruners are a bit dull and squeeze the cane or stem a lot before actually cutting it.

If this is still too costly, then here is a recipe you can try: 3 tablespoons baking soda plus 1 tablespoon of Murphy's oil soap or a lightweight horticultural oil in 1 gallon of warm water. Mix well and let cool. Mix or shake up well again just prior to spraying. Apply this spray every 7 to 10 days until the powdery mildew is gone. Be sure to spray under the leaves as well.

https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm

https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm

It sounds like thrips to me. They can be extremely tough to get under control and many times the use of a systemic insecticide is needed. There are also some sticky yellow pads that you can buy to catch many of the adult thrips so that they cannot make more thrips. They are attracted to yellow. Here is an article that will help as well: https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-thrips.htm