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Try using neem oil. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

If the leaves turn black and curl up, the plant is probably being kept too cold. It prefers warm, tropical-like temps. If the leaves stay green but curl up, that is a sign of too little light. Try and keep the plant in full sun at least 4-6 hours a day for best results. This article will help: https://www.gardeningknowhow.com/edible/fruits/grapefruit/growing-grapefruit.htm

It might need fertilizer. Even planted in fertilize potting soil, they need fertilizer regularly to replenish what gets washed out when watering and just what they use. This article will help:
https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm

Grafting to a sucker may work. I would separate it though and allow it to develop its own solid root system first, otherwise, it would never quite get enough from the mother plant to really do well. If this is not what you want, I would graft to the branch then.

Sounds like you have a scale infestation. Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm

If it was a store bought grapefruit, the fruit you get from the trees will be inferior to what you are use to. Store bought citrus grows on trees that are grafted so that they produce a better fruit. Yours is not, so the fruit will not be as good.

Beyond that, citrus trees can take up to 10 years to produce. It may be many years before you see fruit.

But, they actually make very nice houseplants. Just make sure they have enough water and sun and they are very lovely.