I think the biggest tell that there was a fake gauntlet was that the one in Odin's vault was right-handed, whereas in the comics, and the films, Thanos' gauntlet was left-handed. I think the producers of Thor just threw in the "fake" gauntlet as an easter egg (as are most of the things in Odin's vault, which will likely never be revisited).
As for Thanos' credits scene in AOU, I think it was included for two reasons - to remind fans of who Thanos is (if they've only seen the first Avengers and/or perhaps haven't seen GOTG Vol. 1), and what his deal is with regards to collecting the Infinity Stones; and to bring back the off-shot side-quest that Thor went on, now that Thor was aware of the Infinity Stones' appearances in the MCU, his fate would then be further tied to Thanos and his quest to get them all.
As for the timing - I've heard that the AOU credits scene was retconned to take place not long before Infinity War, probably in the 2016/2017 time frame. Thanos would have already commissioned Eitri to craft the gauntlet for him, but left it in storage until he had a plan put together to start gathering the stones. One theory says that he didn't start his quest until he had learned of the death of Odin (at the beginning of Ragnarok), mostly because by that point, the Nine Realms would becoming unguarded, and so there'd be no protection from the Asgardians (who had the Tesseract, and knew where the Aether was being kept). At that point, all Thanos had to do was brute force his way past Xandar's Nova Corps and retrieve the Power Stone.
Of course, there's the real world explanation that these things just change over time because of different directors, producers, and changes in design. The gauntlet seen at the end of AOU was a bit different than the one we see in Infinity War, which are both different than the one seen in Thor/Ragnarok (which was proven fake anyway), but it's nice that some things can be explained away with retcons.
-Marc.