will a rooted chunk of onion that has grown "leaves" while in my compost, moved to a small pot, regrow a whole bulb?
Probably, given sufficient light and growing conditions, it will produce a flower, and then seed.
With the onions, when you plant a bulb you'll get just "a" bulb, IFF it is fairly small, [(we are not talking about shallots and dividing onions now)] otherwise ,being in its second year, it will bolt, flower and produce seed. However, it will grow thick green leaves before bolting that can be used just like scallions.
If you leave it alone, it will have a very nice globe shaped flower(like flowering alliums). Then, if pollinated will produce a lot of seeds. But saving seeds from an unknown variety is not advised because it might no be suitable to your climate. But you can plant them to be harvested as scallions.
the leaves grew to about a foot tall, then flopped over, still green, then more are coming up from the center