To answer your questions:
- While it is true that Koth of the Hammer was available in the Vault a few months ago and has since been taken out (e.g. is not currently obtainable), according to a note on the official list of currently available planeswalkers:
Given the increasing number of Planeswalkers and Card Sets in-game, we are starting to rotate Planeswalkers and Card Sets in and out of the Vault to put a larger focus on the newer sets and keep the game feeling fresh. But don’t worry, you’ll still be able to use any cards or Planeswalkers you currently have in your collection. You’ll also see all the older Planeswalkers and Card Sets returning to the Vault in the future, and you’ll be able to earn cards from older sets in-game.
Thus, it looks like there is a good chance you will have the opportunity to get Koth of the Hammer sometime in the future, even if you can't right now.
- Based on posts from threads like this one:
Changelog / Balancing
Planeswalker balance changes
- We rebalanced Kiora, Master of the Depths and Sorin, Grim Nemesis.
- Kiora, Master of the Depths
- Her first ability, Crashing Waves, now costs 9 Loyalty (up from 6).
- Her second ability, Swirling Waters, now gives 1/2/3/4 mana to the fetched creature (down from 3/6/12/16).
- Her third ability, From the Depths, now costs 30 (up from 24).
or this one:
Updated for the 1.3 Patch - Major changes in balance: disable supports nerfed, Gideon 1 nerfed, Harbinger nerfed (hallelujah), bounce nerfed (needed, but a bit harsh). Nissa buffs. Angel tomb worthless. Knight of the White Orchid nerfed, now pretty bad.
it looks like cards and abilities have definitely been nerfed (and buffed) in the past, so if Koth of the Hammer becomes truly game-breaking, it is quite possible that he will get nerfed in some way in a patch.