You have a Phalaenopsis. You haven't killed it. Given good care it will grow another flowering spike. There is a mountain of information on this site. Just put Phalaenopsis in the search box at the top of this discussions page to find. Should be some very good complete culture notes written for Hawaii on the net. Here is a link that might be useful: thyme.tyger's orchid

It's possible that an excess amount of fertilizer/wrong type of fertilizer did damage the root system. In any case, mid-July is too late for fertilizing. A late flush of soft growth will not have enough time to harden off before winter. Fertilizing in the first year after planting - or any other time - without a soil test indicating a need for it is never a good idea. The weather conditions you describe are ideal for phytophthora infections, but those that effect the roots would be nearly impossible to diagnose from looking at a leaf. Poor drainage is probably the number one cause of root rot problems, so anything that can be done to improve it would be worthwhile. Do cut back to healthy green tissue and be patient. The presence of buds is a good sign and it's highly likely that the rhododendrons will recover.

Be sure you cut away every bit of soft tissue. If you save partial pads, cut into healthy tissue to be sure you've cut away all mush. Bacteria grow rapidly in the damaged tissue and can sometimes spread to healthy tissue when a plant is stressed or dormant, so be sure to get rid of that stuff. Opuntia species are fantastically resilient and easy to propagate. I have seen Opuntia pads root from between blotter papers in a plant press left over the holidays in a cold, dark botany lab. You needn't keep them damp. They have adequate moisture in the pads to feed new root growth, and they can't absorb moisture through the "skin" anyway, so my feeling is that adding moisture will only increase the danger of rot. Personally, I think it will root just fine if you lay it on top of dry soil. Just be sure the damaged portion scars before you let it get at all damp. Some Opuntia species are very cold tolerant. Others aren't. There is one species that grows wild around Boulder Colorado- Opuntia compacta maybe. That isn't your species, but it's easy to see why you thought it might survive such intense cold. I hope you can salvage some. It looks like one of the more attractive Opuntia.

I don't see why sunlight would have affected the roots. anyway, put it in a bright spot (southern window) and just leave it be for a while. Since most or all of the leaves have died, it's going to be using very little water. That said, cut way back on watering. Do not fertalize either. just give it time and it'll probably put out some new growth. Leaves that are grown indoors can't just be placed in full sun. They're not used to that. You need to slowly acclimate the plant if you want it grown in full sun. New leaves that do grow in full sun will not burn as they'll grow already acclimated to the higher light levels.