Is chicken broth be categorized as umami?

Chicken broth is what water turns into when you boil chicken in it. It does not exactly contain chicken, but it does contain some of the chemical constituents of the chicken.

Yes you can but be sure you cook the chicken (either whole or portions) until it's thoroughly cooked and not still partly raw showing pink or red flesh near the bone. Actually cooking a whole chicken...

Depending on the recipe, probably. But the flavor won't be as intense, so another option is to add chicken boullion cubes to the broth that you have.

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Yes you can, you can cook it, boil it, make a soup with it, or make a stew with it. However it will not improve the taste too much, but it may make it more gamey.