Say I activate Preacher, my opponent in response activates his Mishra's Factory making it a creature and then gives me control of the Assembly Worker. My question is, do I retain control of the Factory when it reverts back to a land? I believe the answer is yes, but just wanted to be extra sure.
Note that as you activate Preacher's activated ability, an opponent of yours has to choose a "creature ... they control" that it will target, if possible, right then and there (C.R. 602.2b, 601.2c, 601.5, 115.1, 108.1). That opponent can't choose a Mishra's Factory they control this way unless it's already a creature. In general, while you're casting a spell or activating an ability, no player has priority, so no other player can cast spells or activate abilities, including Mishra's Factory's second ability (C.R. 601.2a-i and 602.2a-b have no priority window until the spell is cast or ability activated; C.R. 115.1a-b). See also this thread.
In any case, the effect of Preacher's activated ability lasts "[f]or as long as Preacher remains tapped", not necessarily for as long as the targeted creature remains a creature (C.R. 611.2a, 108.1).
EDIT (Oct. 21): Add rule citation. Some rules were renumbered with Core Set 2020.
In any case, the effect of Preacher's activated ability lasts "[f]or as long as Preacher remains tapped", not necessarily for as long as the targeted creature remains a creature (C.R. 611.2a, 108.1).
EDIT (Oct. 21): Add rule citation. Some rules were renumbered with Core Set 2020.