How do Smokestacks of Gnaak and Mirror Realm interact?
By the time Mirror Realm enters the realm, it's already a copy. Thus, it can copy a nearby site, and then promptly loses all abilities of that copied site (but still is the copy, without abilities). See 'Apply Ongoing Effects In Order' in the codex for nitty-gritty details.
Can Mirror Realm copy Rift Valley and enter the realm in the special way, as long as it ends up nearby the Rift Valley that it's copying?
Yes.
What happens if Mirror Realm copies a Rubble?
If Mirror Realm copies a Rubble, you gain no mana. You still control the Rubble, it can be attacked normally, you can play sites adjacent to it, it can be replaced by a site normally, etc. Track it carefully, distinct from other uncontrolled Rubble in the realm.
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