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You can plant it but it won't make other onions. They do get a nice flower and eventually would go to seed, which then you could plant to grow onions in the future.

This is normally caused by too much nitrogen in the soil and not enough phosphorus. Manure and wood shavings are an excellent source of nitrogen but tends to lack significant amounts of phosphorus. By mixing bone meal fertilizer in with this, you have a more well balanced, organic fertilizer. So try adding some bone meal to the soil when planting root vegetables and this should help with the bulbing.

These articles will help get you started: https://www.gardeningknowhow.com/edible/vegetables/vgen/the-layout-of-your-vegetable-garden.htm, https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-garden-design.htm, https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-garden-orientation.htm

You can plant it, but it will not grow more onions. What it will do is grow and then flower. From those flowers, you can collect seeds that you can plant in your garden to grow more onions. Here is more information on growing onion seeds:
https://www.gardeningknowhow.com/edible/vegetables/onion/growing-onion-seed.htm

No question is silly, as this is how one learns. Plant them about an inch deep. The following article should help with growing your onions: https://www.gardeningknowhow.com/edible/vegetables/onion/growing-onions-in-container-gardens.htm

The manure needs to be composted or well rotted before being used in the garden, as otherwise it will burn the newly planted plants. You can mix composted manure and lime, but I would only recommend adding lime if you have had a soil test that recommends that the soil pH is off.