Board Game Arena

I was under the belief that a bloom needed 3 eyes to be alive.

I just found out today a situation where 2 eyes is enough. If you have one normal eye, and a 4-size eye in a trefoil, your bloom is alive even if they occupy the center. The key is to capture with your other color when they fill in three of the spaces.

Are there other situations where a group with less than three eyes is alive? (Other than the typical Go seki due to liberty shortage)

I don't see how the 4size eye is safe. Say this is player A trying to defend this position. Doesn't player A slowly have to fill up the 4 hole with how own other colour stones?

B plays 3 stones (same colour) threatening next turn to take.
A must play 1 other colour to capture.
B plays 2 stones (same colour, over 2 turns) threatening next turn to take.
A must play another other colour stone to defend (if not this then I don't know what..?)
B plays 1 stone (same colour) threatening next turn to take.

There is the obvious situation where the 2 colours can share eyes between them, but I'm guessing you still count that as 3 eyes.