mtg counter target spell cards Counterspell - Single Card Counter target spell. Counterspell | Mystery Booster 2

Yes, the restriction is relevant, but remember this includes equip costs, the hour counters on Midnight Clock, a second trigger with Strionic Resonator, or a copy from Isochron Scepter

On top of this, we also know more characters that are coming to the set, which just seems to be everyone

No, you cant use colorless mana for any color, as colorless mana can only pay for generic costs or specific colorless requirements

Are there other ways we can put the Leviathan Creature type to use

On top of that, there are also a handful of creatures that have lifelink outright: If you are in need of life, Heliods Intervention is an X spell can gain you life equal to twice the amount of X