1) Constructive does not mean more effective with additional copies. It means it buffs/heals rather than removes/damages
2) Each card is usually estimated at 1-3 elemental quanta [3 is used below]
3X

+ X cards -> X Cockatrices <- 6X

+ 1 card
If a card = 3 quanta
6X -> X Cockatrices <- 6X + 3
Fractal or Mitosis are additional costs to break over the 6 card barrier. When X>6, normal methods cost more than 6X
6X + 14 (10

+1 for duo +3 for card) -> X Cockatrices <- 6X + 3
Nerf the Eternal spell to 7

6X + 14 (10

+1 for duo) -> X Cockatrices <- 7X + 3
Costs are equal at X=11
From 1-6 Cockatrice is king
From 7-10 Eternal is king
At 11 Fractal and Eternal are tied
From 12+ Fractal is king
This is niche based balancing and is easy to adjust to many difference examples.
Miracle: 15

+ drain + 1 card for ~80 hp ~= 18

+ drain for 80hp
Heal: 3

+ 1 card for 20hp ~= 6

for 20hp + EM possibility
If the above cards are balanced then an Eternal Heal spell needs only be balanced relative to the most similar card (commutative property of balance)
Set balance at 7 cards
Niche balancing again. This time using already existing restraints.
Heal will be king at 120hp or less. Eternal will be king at 140hp or more.
21

+ 7 cards = Y

+ 1 card -> 140hp
21

+ 6 cards = Y

-> 140hp
39

= Y

-> 140hp
140hp / 39

= 7hp per 2

Buff Eternal spell to 2

-> 7hp