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For a Japanese maple of that age, there is a chance that it is dying from simple old age. It depends on the variety, but if it is a smaller variety, it typically have a much shorter lifespan than the larger ones.

As far as possible diseases, this article has several possibilities that you can use to see if it might be what is affecting your tree: https://www.gardeningknowhow.com/ornamental/trees/japanese-maple/japanese-maple-problems.htm

Bolting a split like that is the common solution for this.

This article, in the section on split forks, will have steps for using a bolt to repair the split:
http://essmextension.tamu.edu/treecarekit/index.php/after-the-storm/tree-damage-and-hazard-assessment/repairing-storm-damaged-trees/

But, that being said, it does not always work. You may want to bring in a tree specialist to look at it to help increase you chance of success.

I would secure the two side of the split back together and hope that they heal. It does not always work, but otherwise, the tree sounds as though it would be a lost cause, so it is worth trying this.

Yes, you can keep it in a container. Japanese maples grow slowly and therefore make a great container tree. You should put it in a pot that is 2" wider than the root ball is currently. You can go a bit bigger than this if you do not want to repot for a few year. Here is some information that will help you: https://www.gardeningknowhow.com/special/containers/how-to-grow-container-trees.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/growing-trees-in-containers.htm

You can keep it in a container. Japanese maples grow slowly and therefore make a great container tree indoors. If you are worried about sap, you can set the potted tree on a plant skirt. You can also set it outdoors during warm weather, when sap is most prevalent.

It could be aphids or a similar pest that sucks the sap and produces honeydew, which could attract the flies. The honeydew is sticky, which may have trapped the flies on the leaves.

I would treat the trees with neem oil. It is organic and very effective on the kinds of pests that suck sap and kill plants that way. Here is more information on it:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm