2021-11-19 : Any enters-the-battlefield abilities of the copied creature will trigger when Mirrorhall Mimic enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

2021-11-19 : If Mirrorhall Mimic somehow enters the battlefield at the same time as another creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

2021-11-19 : If another creature becomes a copy of Mirrorhall Mimic, that creature is also a Spirit.

2021-11-19 : If the chosen creature is a token, Mirrorhall Mimic copies the original characteristics of that token as stated by the effect that put the token onto the battlefield, except it's a Spirit. Mirrorhall Mimic doesn't become a token in this case.

2021-11-19 : If the chosen creature is copying something else (for example, if the chosen creature is another Mirrorhall Mimic), then Mirrorhall Mimic enters the battlefield as whatever the chosen creature copied.

2021-11-19 : If the copied creature has {X} in its mana cost, X is considered to be 0.

2021-11-19 : Mirrorhall Mimic copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below), except that it's also a Spirit. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on. Most notably, if it copies a creature that's not normally a creature, it won't be a creature.