Chondra raised this question and let me answer it in two parts:
- In specific to the verse at 2 Thess 2
- In general.
Yes, 2 Thess 2 is reliably translated by almost every translatory by rendering "apostacia" as "apostacy" in English. This verse gets a lot of attention, largely because....it doesn't fit a lot of people's theology who would LIKE it to say "rapture" here instead of "apostacy". But we cannot fit the translation to fit our theology - we must fit our theology to match the text. There's not one historical reference to using "apostacia" to mean "rapture" in Greek in the Bible or even outside of it, but that won't stop self-appointed experts from telling you it means exactly that.
You are doing the right thing by checking multiple translations. Some of the better, more literal translations are ones you cited too such as KJV, Darby, YLT, OJB (though its not always easy for people to read who aren't familiar with synagogue speach). But even then some of the better ones make mistakes. The KJV tells us that G-d decieves (Ezek 14:9), that men are omniscent (1 John 2:20) and men should be worshipped (Luke 14:10), that Yeshua was "made" rather than being born (Gal 4:4), that the end of the world has already come (Heb 9:26)! And yet there are still people who will tell you it is error free!!!
Sometimes mistakes in early translations simply get repeated in later translations out of familiarity with the English reading. Sometimes people let interpretation get in the way when they translate. Some verses are ambiguous and some have several shades of meaning that can't be rendered into English with a single word.
And sometimes due to a combination of these things coming together in the same sentence, you get an English translation that isn't even close to what was intended. One of my favorites is Gen 49:6, and at http://www.messiahalive.com/genesis.htm at part 12, last video on the page at 12. Vayechi (Gen 47:28-50) - All of Israel comes to Egypt , there is an explanation as to why most translations have something different, and they ALL GOT IT WRONG. Shimon and Levi did not "digged down a wall" as the KJV says, using horrible English for an allegedly error free version. They did not "hamstrung an oxen" (NIV) or "lamed oxen" (NASB) [Is there an ox anywhere in their stories????]. They did "eradicate a prince" (YLT), but that's not what the text says, although the YLT got closer than anyone. They did "make barren a lineage", which is what the text is saying in Hebrew, but even the best translations got that one wrong.
If you can't read any of the Biblical languages, using several translations is a good start. Learning them is better. And don't stop at an introductory level; make certain you can read a Hebrew book without an English translation before you try and analyze the Hebrew of the Bible. Otherwise, you might end up with some weird results. Lots of damage has been done by people who knew just enough to be dangerous. It can be hard getting started, but once you get there and can read without help from dictionaries and other linguistic aides - WOW does it make a difference!