The best info I can find indicates that it grows best planted fresh from the seed heads, so probably in late summer or early fall. I suspect that varieties of doubtful hardiness would go best in a cold frame, while those fully hardy to your area could go directly into the garden. Small varieties will probably bloom in 2 years, while the giant ones will probably take 4-5 years to bloom.

Search "Alliums from seed" for previously answered questions.

Why not try leaving a mature seeded head resting on a pot of soil sunk into the garden. They seed themselves that way. So perhaps they won't notice you did it deliberately!!

Probably easiest. I understand the hardiest kinds need to spend a winter under snow--or some facsimile thereof--to sprout anyway.