Thanks for the invite. It is a great question.
Beautiful art too.
3|4 stats deserve a cost of 3
3 (Attack) + 0 (0<HP<6)
1|6 Venom Upgraded stats deserves a cost of 3-4
1 (Attack) + 1 (5<HP8] +3 (Venom) -[1-2] (Upgrade)
Now the question of "Does Mercury modify the above up or down and how much?"
I personally think that Mercury does increase the cost because it adds a benefit.
But how much of an increase is trickier.
Also with decks that are designed to win in 24 cards or less, this card's casting cost becomes irrelevant.
Since the decks that would benefit most from this would never need to cast this card, I think balancing it as is might be impossible. However what about it have a skill that inserts it at the bottom of the deck?
Casting Cost: 1|0
Effect:
:Mercury:Return this card to the bottom of your deck and draw a card.
Attack: 0|0
HP: 5|7
On a side note:
I fully agree with you that Gravity should be the element of certainty and Entropy be the element of luck (sometimes biased sometimes fair)
In general I believe that certainty is only as valuable as the certain result just as random is only as valuable as the average result.