Also, a small point on Amoeboid Changeling. You can use it to remove Precursor Golem's Golem type, but it requires the third ability which will make it not have any creature types. You can also make Precursor Golem an Elf with it, but that means using it's second ability which will still leave it as a Golem, as well as giving it all other creature types, not just Elf. You cannot use Amoeboid Changeling to set a creature to only having 1 creature type.

To clarify further, none of Zaxara's abilities work form the command zone. You won't get Hydra tokens until you actually cast Zaxara. Some commanders, such as Edgar Markov do have abilities that function from the command zone, but they specify it in the ability.

Your opponent reveals cards until they reveal a land, then put those cards (not all other cards revealed) into their graveyard. Because the land was revealed, it goes to the graveyard. Consuming Aberration will always mill at least 1 card (unless their library is empty). Compare to Tunnel Vision which specifies that it mills the other cards and leaves the named card on top.