Naturally the mantid would have overrun them eventually. The trolls would have awakened the thunder king regardless of horde/alliance interference and they would have died, etc.
The events caused in 5.0 were resolved by us in 5.0 and conveniently we saved them from another threat at the same time.
The mantid "overrun" has been attempted many times before but never succeeded. The Pandaren are quite capable of handling them. You simply see places like the Stoneplow village unprepared for the attack because the Mantid attacked ahead of schedule. Other places like Townlong the Pandaren are struggling because the Shado Pan are stretched too thin, and slowly recovering from Sha posession themselves. Once again because of the Sha caused by the Alliance/Horde in the first place.
The Zandalari are outside invaders, just like us, and this is nothing the Pandaren caused. What lesson did you teach the Pandaren by this? You just reinforced Taran Zhu's idea that outsiders are all dangerous. Then you have Jaina/Lor'themar possessing the Thunder King's weapons and "reassuring" the Pandaren that they will "put them to good use", yeah right.
It's incredibly arrogant for Alliance/Horde to think we just come trash Pandaria, and then think we save Pandaria when most of the problems are caused by us in the first place.