I don't find that a real theory. It's just some possibility with nothing to prove or to support it.
There is some kind of connection between the dragons and Zeref, so much is certain based on what we've seen in the plot. However what that exactly is unknown though saying that they were controlled and that they trained Natsu & co to defeat Zeref seems highely unlikely as that's a huge contradiction. If they're being controlled how can they start training people to defeat Zeref? That would not be possible if they're being controlled. Also that would make the dragons look like a bunch of wimps as it's pretty much "were to weak and are being controlled so we train a bunch of kids to save us" =/
We already know Zeref was sealed, more precise he's asleep according to the manga. So unsealing of awaken is pretty much the same in this context. The dragons left in the year 777, unless we've got the old prophecy-plot or an evil wizard who has a number-fetish I don't think just because of a plausible seal they would leave their childen for years.
We know way to little about Zeref, dragons and dragon slayers to make a believable theory. The timeframe still doesn't match. Zeref is centuries old, still emo-kid knows Natsu, while Natsu doesn't know him and for some reason Natsu and Gajeel were deemed over 80 years old while they still had a normal biological youth.
It's possible, but it's still just a guess than anything else. We don't know enough. Saying that Natsu & co have been reincarnated for centuries over and over again is as much possible as what you're saying.
