My interlinear (Berry's) reads "monon ho katechon arti heos ek mesou genetai". Berry translates it as "only (there is) he who restrains at present until out of (the) midst he be (gone)".
Seems the Greek is saying "one who (or "one that") restrains currently until out from the midst he comes to be". I am not sure what genetai refers to: is it one who restrains? Or is it the one to be revealed (next verse, the lawless one)?
In other words, is Paul saying tat which restrains will come to be out from the midst? Or is he saying the restrainer does so until the lawless one is come out of the midst?
Seems the simplest reading is that there is something restraining the revealing of the lawless one until that lawless one comes out of the midst.
Another question is "out of the midst of what"? It could be the phrase means out of the midst as though in between two things (thus the translation "out of the way"), although I suppose it could also mean out of the midst of the apostasy / mystery of lawlessness.
I do not, however, see that the restrainer is the lawless one. The text says there is a restrainer UNTIL something comes "out of the midst", and THEN the lawless one is revealed. Therefore it looks to me like the restrainer is restraining the revealing of the lawless one.
The restrainer appears from the immediate context to be holding back the mystery of lawlessness, until he or someone/something comes to be out if the midst. "Comes to be" can be taken to mean "originates out of the midst", OR if I am not mistaken it can serve as a copula joining "he" with out of the midst. This would mean not that it comes from out of the midst, but that it enters a condition of existence out (side) of the midst.
The word genetai is a form (conjugation) of the same word translated "became" in
John 1:14. Thus he/it "becomes" or "comes to exist". The STATE which he will "be" is "out of the midst". Therefore, until I see more conclusive information on the phrase ek mesou genetai, showing that it indicates "ek mesou" is the SOURCE from which he "comes to be", I have to default to the translators who took ek mesou to mean the condition into which he comes to be.