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Pothos is pretty much the kind of plant that will make its way whatever the conditions. Don't even concern yourself with room for roots - pothos will grow according to whatever room it has, and it doesn't need much to start growing up the trees. The one thing it doesn't like is too much sun. If the spot is too bright, its roots will set off on their own search for a more hospitable spot. I've never heard of pothos roots damaging pipes, because the roots are pretty much on the surface. However, any plant's roots have the capability to "damage" pipes if the pipes are where the roots can reach them.

First of all, this is not ivy, it's a pothos. If you try to look up information on ivy, you'll be very confused. A common name for pothos is devil's ivy, but ivy and pothos are in completely different families, much like alligators and horses.
Anyway, to answer you question about your pothos, most of the leaves are probably goners, but only time will tell if the roots were killed. I would place the plant in a spot with moderate light, make sure the soil is lightly damp but not wet, and allow it to rest until you can see if any leaves are living. Cut off leaves that are brown or withered, and if whole vines go, cut those off back to the soil. If there's life in the roots, they should sprout new leaves in a few weeks. If the roots were frozen, nothing you can do.

Pothos will sometimes put out darker leaves depending on the amount of light they receive. If the plants you started with were lighter or "marbled" they were probably grown in optimum light conditions - indirect, bright light. If your setting is darker, the plant will use it's energy to grow, it can only generate energy from the green portion of it's leaves if it is not getting enough light, it will compensate by putting out mostly green, darker leaves.
As to the longer strands, this is a normal growth pattern of the Pothos. If you want the plant to produce more from the base you need to trim off the longer strands.

Since this is just a few leaves and your plant is doing very well, I do not think there is cause for concern at this point. With age, some of the lower leaves will yellow and drop as a normal part of growth.

For more information on the care of pothos, please visit the following link:
https://www.gardeningknowhow.com/houseplants/pothos/pothos-plant-care.htm

I would not know of a way to do this.
Perhaps just planting the two plants together would give you the effect your looking for.

Brown spots 9and edges) in houseplants typically are caused by watering issues. I would first check to see if the plant is rootbound, which could prevent the roots from taking up water. That is most likely the cause. Additionally, it could be a fungus or even pests. Neem oil can help for both of these.