A deep voice thundered from the sky

A bolt of lightning struck my brain

When I came to, I had new understanding.

regarding...

Shrine of Taal -

Once the card is triggered and a target unit for the effect chosen, does the power gained change if the number of deployments in the unit's zone changes or the unit is moved to a zone with a different number of deployments?

the Shrine counts the developments and the unit gains power at the time the effect resolves, and then the power gain lasts until the end of the turn. The number of developments changing would be akin to the Shrine itself leaving play before the end of the turn, since the effect was triggered and resolved it does not change.

Twin Tailed Comet-

what happens if the card is played and targets a variable cost card like Flames of Tzeench?
Does it copy the effect as played by the original caster (he inflicted 3 damage by paying 3 resources, so the copied effect does 3 damage?)
does it have no effect because you dont copy the variable (X) cost paid by the original player and you cant pay any additional resources.
or can you pay a variable cost (x) of resources to actually do damage with the copied card, and this variable cost can be different than that paid by the original caster of the card?

"0" is being paid for the cost, defining X as 0 in the effect.