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When deciding how large of a tree to purchase, it is really a personal choice that is made after understanding the pros and cons of a younger (smaller) tree versus an older (larger) tree.

Smaller containers will have younger trees that are less expensive. Younger trees also have a better chance of surviving the shock of being transplanted. But they will take a bit longer to reach mature size.

Trees in larger containers are older. The look more impressive when first planted but they are more perceptible to shock. The older a plant, the more likely it is to go into shock when transplanted. Older trees are also more expensive because they need more care from the nursery.

I personally opt for younger trees. They catch up in size quickly and you are less likely to lose them to shock. But you should look at your own circumstances to decide what is best for you.

There might be some hormone sprays available that would help prevent fruiting. Otherwise, changing the balance of fertilizers you give the tree might help: phosphorus fertilizers tend to promote fruiting. See these articles for more information:

https://www.gardeningknowhow.com/ornamental/trees/tgen/fruit-tree-sterilization.htm

http://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/preventing-or-reducing-fruit-ornamental-trees

I checked some Michigan weed guides and didn't see it, so I would leave a couple and let them flower. Usually once it flowers, it's much easier to identify. Then you can check some online wildflower guides according to the color of the flower. Weeds are also easier to identify with the flower. Also look around your yard or a neighbor's yard for a tree or plant with that leaf shape that may have dropped seed.

Here is a Google link with lots of vine photos - maybe this will help:
https://www.google.com/search?q=vines+invasive&client=ms-google-coop&cx=009296322515861343460:jftykenwinc&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjL3aXx8aziAhWHsJ4KHQfLC64Q_AUIDigB&biw=1366&bih=619

Also, you may want to contact a local nursery or plant authority such as a university horticulture department to help ID this plant.

Adding soil is just fine, as long as you don't cover the base or crown of the tree.

These are very hardy trees and will not be killed off easily.

This is common under Hackberry trees, as well as many other species. Sap will leak from the tree and onto anything under it. This sap will contract sooty mold and begin to turn black.

Here is an article that will help you to remove it:

https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-remove-tree-sap.htm