Hey Caffus - there is actually no Koine Greek word that shares our concept of 'eternal'. It is important to remember that interlinears are still translations, and can suffer from the same inaccuracies as a traditional translation. They give the appearance of greater accuracy owing to the 'word by word' set up, but they can still translate individual words incorrectly.
The Greek phrases that have become translated as 'eternal' or 'forever' in English tend to be some variation of εἰς τοὺς αἰῶνας τῶν αἰώνων - which literally would be something more akin to "to/into the ages of the ages". These Greek phrases are all based upon the Greek word αἰῶν from which we can the English word eon, or age. So all of these phrases are based around the concept of ages. I don't think there is a truly satisfactory translation, as εἰς τοὺς αἰῶνας τῶν αἰώνων seems idiomatic to me, and the literal translation is rather clunky. Unfortunately, modern Greek now uses αιωνιος for eternal, which confuses the matter further. However, modern Greek was to some degree reverse engineered over the past century or so, and either way, languages change over time.
Ultimately, if the Bible writers had really wanted to convey the sense of 'forever', there are better words they could have used - ἀδιαλείπτως (ceaselessly/without ceasing) comes to mind. Also, it is worth remembering that before the church moved from primarily Greek language based to Latin based, there were MANY known universalists such as Origen, who studied and expounded the Bible in great depth. In other words - people for whom Greek was their mother tongue! So it is not just a case of us assessing Greek as a second language, but knowing that there have been Universalists aplenty who spoke Greek every day! For me, it is no coincidence that the rise of belief in ET was concurrent with the transition from Greek to Latin as the main language of the church.