I'm not sure whether I read carefully enough, but I think nobody suggested the first translation that came to my mind:
"for the mystery of lawlessness is already at work, but only until he who possesses it is taken out"
Now i dont know about the context of the phrase, and i dont know what "the mystery of lawlessness" is supposed to be, but that seems to me the most correct translation. The "but" being optional, but useful, I think, to convey the contrast/opposition expressed by "μόνον". I don't know. Would that sentence work better for you if it started with ἀλλά μόνον, instead? I don't understand how else μόνον would make sense there. I mean, what is it supposed to be defining, "ὁ κατέχων"? It can't be... Furthermore, with my translation there is no need to assume a missing word. I'm surprised nobody translated "μόνον" as "only".
II. neut. as Adv., μόνον alone, only, “οὐχ ἅπαξ μ.” A.Pr.211, etc.: freq. with imper., “μ. φύλαξαι” Id.Supp.1012; “ἀποκρίνου μ.” Pl.Grg.494d; so “μ. Κράτος συγγένοιτό σοι” A.Ch.244; “μὴ 'μὲ καταπίῃς μ.” E.Cyc.219, etc.; ἐὰν μ. if only, Arist.Pol.1292a3; οὐσίαν . . , οὐ χωριστὴν μ. only not separable, Id.Metaph.1025b28. http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dmo%2Fnos
Oh, and I just had another idea. I'm just brainstorming here, really, so bear with me. Bill cautioned me in an email about κατέχων expressing some sort of violence. So I thought what if "μόνον" meant "only", but... were defining ἐνεργεῖται. Now ἐνεργεῖται is not a strong word. What if the author is trying to express a contrast between the plain ἐνεργεῖται and a more powerful word he could have used? So my other suggestion.
"for the mystery of lawlessness is already at work; just "at work" until he who is constraining it is taken out"
implying that when he's taken out, that mystery will not just be "at work", it will be about to completely take over the world.