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Leggy plants are normally the result of too little light. You should be able to grow it fine in these boxes. Though, depending on the type of plant, I would recommend placing it in a sheltered location until it becomes stronger. You can plant it deeper if you wish and place it in an area with more light.

As long as the soil is not too wet or soggy and does not remain this way for too long, then this should be fine as you would normally water them anyway. I would, however, hold off on the watering until it dries some.

The following articles should help: https://www.gardeningknowhow.com/special/containers/how-to-grow-container-trees.htm, https://www.gardeningknowhow.com/ornamental/trees/tgen/growing-trees-in-containers.htm

Many flowers can go into coco liners. Whether or not the plant is good for putting into the liners is determined by, first and foremost, light; and after that, by the amount of water they will get. This article gives you a run down of different flowers and plants you can use in hanging baskets: https://www.gardeningknowhow.com/special/containers/hanging-basket-plants.htm
Here's some general infor on coconut liners: https://www.gardeningknowhow.com/special/containers/coconut-planter-liners.htm

Too much nitrogen typically shows up as flowering plants that have lots of green leaves but no blooms. It's almost impossible to create nitrogen toxicity. The amount of nitrogen contributed to the soil by coffee grounds is rather low, but it's all good. Check out this article on using coffee grounds in the garden : https://www.gardeningknowhow.com/composting/ingredients/coffee-grounds-gardening.htm

This article contains the information you're looking for: https://www.ctahr.hawaii.edu/oc/freepubs/pdf/GHGS-28.pdf