What I would do? I wouldn't disturb the root ball. I would cut the dead stem off at ground level and leave all the remaining stems together, I would pull off the diseased leaves. I would make sure the potting soil was wettable (sometimes it gets water repellent after drying too much.) Then, I would leave it in a spot with filtered sun and make sure it got watered daily. When it starts to look healthy and grows a bit, move to more sun and eventually to the ground. An 8 inch pot will dry out very fast in your heat. I am thinking that you might want to put some bark mulch on top. I do that will all my potted roses. Most folks don't save these 4 inch potted roses, but I have some very pretty ones that have remained here in my garden for years. Good luck!

I'd plant it in the ground if you can but if not put it in a bigger pot! An eight inch pot isn't big enough for even a mini rose. Remember, mini only refers to the size of the leaves and blooms, not the size of the plant. Besides it probably could use some fresh soil by now anyway. Soak it well the day before you do either of those and keep the root ball intact when you transplant it. I wouldn't try to separate them at this point. They've been growing together for too long and you'll lose too much of the feeder roots trying to pull them apart. Don't use any chemical fertilizers on it until it's grown some but you can add something organic like fish emulsion. And yes, sorry to say this but, I'd pinch off that bud and give this poor thing a chance to recover. Cut out any dead canes too. Put it some place where it gets dappled or morning sun only until it looks better. Give it some time and it should recover nicely.

If you decide to keep it in a pot your best bet for wintering it is in an unheated garage. But if you garage it you still have to water it once a month all winter. Even when it's frozen and dormant! I know that sounds crazy but it's dormant, not dead, and desiccation will kill it quick. Learned that one the hard way myself. The other thing you can do is sink the pot completely into the ground for winter. That way the rain or snow will keep it watered for you.

Thank you both for the excellent advice. Seil, I was just thinking that it may be better off in the ground...my main concern with the pot it's in right now is that it's so small that the soil dries out way too quickly in full sun. Planting in the garden would solve that particular problem! (Though, I am a little concerned about its powdery mildew problem spreading...hopefully it'll put out clean new growth so that I can pick off the diseased leaves...which are pretty much all of 'em at this point).

Tomorrow I will put both of your suggestions into practice. Wish me (and my poor little rose) luck! :)

Claire