"1. Can I still plant them somewhere else? I don't have much room, but I may be able to squeeze them in somewhere."

If the bulbs are still alive, you can plant them. You can plant them in pots if there is no room in the garden.

"2. How should I store them? I store my tender perennial roots in peat in a dark area of my unheated basement. Would this work for alliums as well?"

It is generally recommended that you store in the same manner that you store onions and garlic. However, they can be frozen as well, if the bulbs are cold-hardy. This may not be important to you, since they should only be frozen if you plan on planting them in the spring. The freezing simulates a winter outside. This is useful if you didn't get them planted in the fall.

"3. If I do store them, will they be any good in the fall?"

If you left them unplanted until the fall, I highly doubt if they would survive that long, but maybe they would.

I would plant them out in the spring anyway. I doubt they will last if left unplanted until fall. I have done this with many other bulbs that one would typically plant in the fall such as tulips or daffs. I have not had any problems.

You may as well plant them and see what happens. Odds are they won't last if not planted.

1-what kind of Allium do you have ?
2- If onions, then you can find a spot to plant them real close, say one every one inch. You then thin, pull and use them as scallions/green onions. This way in one square foot you can plant 144 of them (12 x 12 = 144)