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Here is a link with help on when to harvest. Growth rate will vary with environmental conditions.

https://www.gardeningknowhow.com/edible/vegetables/cabbage/harvesting-cabbage.htm

I have heard of a cabbage plant producing second heads after the first head was harvested. If your cabbage simultaneously produced many heads on one plant that is an interesting discovery! You could try saving seeds from the plant if it prudices any.

If you didn't grow this plant from purchased seeds this year, it's possible that it is a cross between ordinary cabbage and a brussels sprout plant, since these are two varieties of the same species and can cross-pollinate.

Yes, all parts of the cabbage except the root are edible.

Cabbage can continue thriving in light freezing temperatures, and even as low as 26 degrees, even if they've been snowed on. Temperatures lower than this, or sustained temperatures below 28 degrees, can damage the leaves, so they should be harvested before this happens. Some cabbage varieties are even more cold hardy than this.

It looks to me like fungal problems, specifically a fungal disease called damping off. This article may help:
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/what-is-damping-off.htm

This article will help you.
https://www.gardeningknowhow.com/edible/vegetables/cabbage/protecting-your-cabbages-from-cabbage-maggot-cabbageworm-and-cabbage-moth.htm