I would like to know how to grow scallions.
This article should help with that: https://www.gardeningknowhow.com/edible/vegetables/onion/growing-scallions.htm
Do scallions come back every year? What is the purple flower on the top of scallions?
If they are not dug up, they can come back. The purple flower is just that, the flower of the plant. If you let it go, it will produce seeds.
What is the average seed viability for spring onion seeds (old and new) and what might affect the viability of seeds?
This article will help you: https://www.gardeningknowhow.com/garden-how-to/seeds/planting-old-seeds.htm
Seeds for onions are normally only viable for a year.
How do you grow green onions over and over?
Normally, you would replant them each year. These articles will help: https://www.gardeningknowhow.com/edible/vegetables/onion/growing-scallions.htm, https://www.gardeningknowhow.com/edible/vegetables/onion/harvesting-scallions.htm
Can you cut off the sets before they bloom and plant them?
You shouldn't allow them to bloom (or bolt) as this will lessen the plant's productivity and may cause a bitter taste. This article will help with bolting in members of the onion family: https://www.gardeningknowhow.com/edible/vegetables/onion/onion-bolting.htm For information on planting scallions, this article will help: https://www.gardeningknowhow.com/edible/vegetables/onion/growing-scallions.htm
Do you thin scallion? I have groups of them coming up from one bulb.
If you are sure it is from one bulb, then I would not worry about thinning them. But if it is several bulbs bunched together, I would thin them so that the strongest plant will do the best.
How far apart and at what depth should I transplant the scallions to produce onions?
Here are some articles on regrowing green onions that will help you:
In the garden: