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You are in zone 5a. The average last frost date is between May 1-10.
These dates are guidelines and you will need to watch the weather trends.

When your seedlings have generally reached 3 to 4 inches in height and an established root systems.
Bulbs can be directly placed in the garden in the Spring.

Remember that all seedlings and plants need to be hardened off before planting.
Here are some articles that will help you.

https://www.gardeningknowhow.com/ornamental/bulbs/allium/drumstick-allium-flowers.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/how-to-harden-off-your-seedlings.htm

Almost everything in the onion family is extremely hardy. Anytime you would like to transplant will be ok. It is best then they are done flowering though. Here is an article that will help:https://www.gardeningknowhow.com/ornamental/bulbs/allium/allium-post-bloom-care.htm

You can, but I would recommend treating the soil first. Make sure to apply dolomitic lime, and wettable sulfur to the soil to rid of any fungal growth as well. To get rid of the pest, boil some hot peppers, and a little rosemary oil. If you have garlic handy, then put this in as well. After boiling for 10 or 15 minutes, let it cool, and apply this to the soil. This will get rid of insect pests. After this wait a week, and your soil will be ready to plant anything in.

This collection of articles will give you a lot of good information: https://www.gardeningknowhow.com/special/organic/

You can plant these in, either, spring or fall. I prefer to plant them in spring, though. Here is an article that will help: https://www.gardeningknowhow.com/ornamental/bulbs/allium/drumstick-allium-flowers.htm

Most Alliums are quite cold-hardy and can grow outdoors down to USDA gardening zone 3. Plants in containers are more exposed to the cold, but even so, these are very cold-resistant bulbs, so they should be fine outdoors.

Nothing much you can do; mother nature calls the shots. If you had seed pods, you had blooms. As the old song goes, you can't have one without the other. They may have been short-lived or lower than usual due to weather. Are you confident that you planted them at the correct depth?