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If it is rootbound, then you need to repot it. It will not be able to take up enough water if it is rootbound and this would cause the leaves to fall off. This article will help you with repotting: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm If you would like to transplant it in the ground, that's fine too. This article will help with its overall care: https://www.gardeningknowhow.com/ornamental/shrubs/holly/growing-holly-bushes.htm

These plants are not normally grafted. Berries only produce on female shrubs, and there must be at least one male nearby for this to happen. If you have two shrubs and one produces berries while the other does not, then you have one female and one male. If you want additional berries, you'll have to get another female or you can take a cutting from the one you have. These articles should help: https://www.gardeningknowhow.com/ornamental/shrubs/holly/propagation-of-holly-shrubs-with-holly-seeds-or-cuttings.htm, https://www.gardeningknowhow.com/ornamental/shrubs/holly/how-to-tell-the-difference-between-a-male-and-female-holly-bush.htm

Are you sure they are hollies? We have 3 types of hollies here - American holly, English holly, and Japanese holly. I would suggest that you cut off a shoot, one good and one bad section, with and without leaves, and take it to a nearby Nursery, or Arboretum - Botanical Garden, or Agricultural - Horticulture college or school and ask the people there. Its easier when you can see a sample.
Or maybe someone can answer this question better than I.

I would recommend treating the bush with Neem oil, as it will take care of both the scale insects and the sooty mold issue. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm
https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm

It sounds like it may be tar spot. This article will help you learn more: https://www.gardeningknowhow.com/ornamental/shrubs/holly/holly-problems-holly-leaf-spot-or-holly-tar-spot.htm

There are a few things that can cause this. Holly scorch is the most common. This happens when the weather changes too quickly. Newly planted shrubs may be more susceptible. The plants should recover from this on their own. Give them a good dose of fertilizer to help them get back to full health.