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It would be best if it had holes, simply because it makes taking care the plant easier in terms of making sure it is getting the right amount of water.

That being said, you can use a pot without holes as long as you are careful about watering. Just don't add too much water to it and the plant will be fine.

While it roots easily in water, it is always better to root cuttings in soil like potting mix, perlite, vermiculite, or other solid medium. Some plants that are rooted too long in water simply learn to live in water and cannot live outside of an all water environment. Some examples of these plants are pothos and philodendron. Once plants develop enough roots to survive on their own, you should move them to soil. If yours is not doing well, it could be that the soil is either too wet or too dry. For more information, this article will help: https://www.gardeningknowhow.com/houseplants/pothos/propagating-pothos.htm

This is commonly caused by over watering. When you go to water, feel the top inch or so of the soil. If it is dry, water the plant, if it is not, do not water the plant. This will ensure you are watering to the plant's needs. If you feel that you have not been over watering your plant and would like additional reasons as to why it may be turning yellow, this article will help: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

These are probably mealybugs. This article will help you: https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm

I would just give it some time. These are very resilient plants, so I think you have a good chance of it regrowing its leaves. You can trim off 4-6 inches off a vine and put it in water to root as a precaution, but it is likely the mother plant will regrow its leaves. Continue to water, but only water it when the soil is just dry to the touch on top. If it is not getting much light now (as healthy pothos can be happy in low light situations), I would move it somewhere a little brighter. Keep a sharp eye out for pests, as they will be attracted to a damaged plant. You should see signs of new leaf growth at the old leaf nodes in about 1-2 weeks.

Yes, you can place pothos outdoors in summer; however, be sure to locate it where it will not be in the direct sun during the hottest part of the day. In other words, morning sun no afternoon sun, as this could quickly scorch the plant. I would not let it be in temps that are much lower than 50 F. If it will be lower than that, bring it inside.