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Yes, they can be as long as they have not bloomed this year already. If they have, you can still plant them out in the garden, but it probably will not be until the next year that they bloom.

You may want to look around to see if there are some drafts affecting it or place them somewhere a little warmer. Cooler temps would slow down the growth on them.

You may be keeping them too wet. Water them only when the top of the soil is dry to the touch.

Here is information (at the bottom) on making the amaryllis rebloom: https://www.gardeningknowhow.com/ornamental/bulbs/amaryllis-hippeastrum/amaryllis-care-instructions-how-to-care-for-an-amaryllis.htm

The white powdery was probably powdery mildew. Here is more information on that:
https://www.gardeningknowhow.com/problems/get-the-cure-for-powdery-mildew.htm

Yes, it can. Other popular choices for this are daffodils, paperwhites and hyacinth, but really any spring bulb can be forced indoors this way.

You did nothing wrong. Amaryllis bulbs are typically shipped with a bud or two ready to grow so that you get a bloom almost immediately. But sometimes the grower mistakenly packs a bulb that is not budding or the bud simply gets damaged in shipping and does not grow. You can keep them for the year and they will bloom in the spring. Here are directions on doing that: https://www.gardeningknowhow.com/ornamental/bulbs/amaryllis-hippeastrum/amaryllis-care-instructions-how-to-care-for-an-amaryllis.htm