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You can either bolt or tie the split together. It does not always work, but there is a chance it will. If you bolt it back together, avoid putting the bolt through the heart wood.

It sounds like the tree may be getting wilt. This article will help you with that:
http://aces.nmsu.edu/pubs/_h/H160/

No, sealing the crack can trap disease against the wound and will make it worse. Instead, try treating the wound with a fungicide, which will help keep pests away. If the crack is bad enough that you worry the tree may fully split, you can use bolts to hold the two side together and provide additional support to the crotch of the branches.

The only thing to do with wisteria pods is grow more wisteria, as they are poisonous, you cannot do much else with them. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/vines/wisteria/grow-wisteria-seeds.htm

As for mimosa pods, make sure the pods are completely dry, like paper. They germinate easily without stratification (cold treatment). Plant the seeds in a pot of moist potting soil about 1/2 inch deep. Germination occurs in 10 days. As with wisteria, All parts of this plant can be toxic and should not be ingested.

NO, this tree is not worth the trouble! While mimosa trees have a pretty bloom, they are very brittle and very messy. Most people consider mimosa trees to be a weed. You probably will not find a mimosa for sale If you have access to one in the wild, tag it and dig after all the leaves have fallen. Dig a 10" rootball for each 1" in tree diameter.

This article should help with that problem: https://www.gardeningknowhow.com/ornamental/trees/tgen/weed-trees.htm

Yes, pruning off the branches close to the ground should encourage more growth higher in the tree. For more information on the silk mimosa, here's an article; https://www.gardeningknowhow.com/ornamental/trees/mimosa/growing-silk-trees.htm