Speaking of the O5, I don't understand why time travel in the Marvel Universe works selectively, where sometimes it causes a catastophic distruption in the timestream (as seen in the opening Kang storyarc in the original Allan Heinberg volume of Young Avengers) and other times it doesn't.
When I first started reading Marvel, the general understanding was that you could not change the past. If you went back in time and altered an event, you history was the same, but you created a new time stream that went on from there.
That all really started to change after AoA, when the main Marvel timeline was actually altered. The M'kraan crystal had to be used in order to set things right.
Since then, things have become more and more wonky. The O5 being brought to the present probably damaged things exponentially.
I think Age of Ultron was written in part to address what you are talking about. AoU established Time as a living entity, and that travel through it creates cumulative damage. Each new incursion through time makes thinks worse and more unstable. The people on Marvel Earth have been doing a lot of this, and it is finally catching up. AoU closes with them saying they are lucky it still exists in a linear form.
It is my belief we will get a Marvel event sometime in the future that deals with this specifically, and will establish a set of rules for time travel, along with fixing the current predicament. In the short term, broken Time allows for things like the original X-Men to exist concurrently with their modern counterparts, and the universe will not implode because of it.
It is not supposed to add up, right now.