Do I still need to meet threshold requirements and pay additional costs, e.g. discarding a site for Craterize?
Yes!
How does Blood Mana interact with reduced costs?
The same cost you would pay with mana is paid by your life instead.
Can I pay more life than I have? What if I'm on death's door?
You cannot pay more life than you have. If you're on death's door, you won't be able to pay any life and Blood Mana won't help you.
What happens if the next spell I cast is Twist of Fate?
Market
$9.47
Low
$9.37
Median
$12.26