I tried growing red onions in containers but I did not have enough depth of soil so the onions did not grow to a usable size. Can I replant them either now or next spring?
Yes, you can do this. Add some phosphorous to the soil to help get bigger onions next year.
Should they be bundled? Or spread out individually to provide maximum ventilation?
If you can, spread out is best. Some people like to use panty hose and tie a knot between each one. They can then just cut one off at a time when needed and the onions get maximum ventilation.
I was given a large plant grown from a large bulb. They said it was an onion plant. It flowers but not like a regular onion. Hope you can tell me the name.
It's likely an allium plant, of which onions are related. Here is more information: https://www.gardeningknowhow.com/ornamental/bulbs/allium/growing-alliums.htm
Can I store spring salad onions?
Typically, the only way these can be stored is to freeze them, and then you would only be able to use them for cooking, such as in soup or a stir fry. They don't really store long term other than that.
For your short term storage, you can keep them fresh longer by wrapping the bulbs in a damp paper towel and then placing this in a plastic bag.
Planted what I thought was onions last June and they never got large or looked like onions, so I'm thinking they were shallot plants because they are small clusters of bulbs. They've been in my garden since I planted them, 9 months, and I've only used the green tops like chives. They are nice and green and there are lots of flat bulbs. Can I still use them right out of the ground like onions/shallots? Are they still good? Do they have to be dried first?
Use them. Pull, clean and use. Tthis article should answer other questions you might have about shallots: https://www.gardeningknowhow.com/edible/vegetables/shallot/tips-for-growing-shallots.htm
Sowed onions in seed tray in end of December, transplanted into individual pots in middle Feb. Now they seem to have stopped growing.
Hopefully this article will help: https://www.gardeningknowhow.com/edible/vegetables/onion/growing-onions-in-container-gardens.htm
I was told folding over the seed stalk to frustrate seed formation directs more of the plant's energy to bulb formation. Is there any truth to this? Sometimes, the stalk breaks entirely, probably not a good thing. What about snipping off the flower head at its apex?
If your onions are starting to flower, then they are past their prime. Once the onions start flowering, most of the sugars rush up to the flower making the onion kinda tarty not so much sweet. good luck! They make pretty blooms
These articles should help: https://www.gardeningknowhow.com/edible/vegetables/onion/onion-info-tips-for-growing-big-onions.htm
https://www.gardeningknowhow.com/edible/vegetables/onion/onion-bolting.htm