So considering the big vent on the backside of Rog 2 , will the device survive if the water gets inside through the vent ? Is there any safe mechanism Asus did to prevent such ?
No, it won't survive. You can see the teardown video from JerryRigsEverything
We do not recommend using ROG Phone II during a water fight. It would probably survive some minor splash but there's no guarantee. Minor rain shouldn't be a problem but if the water indicators inside are colored when we open it up, then the warranty is voided.
With the vent in the back being open to the internals, what level of ambient humidity would become "unsafe" (either for the water indicators, or the phone itself)...?
Humidity will always find its way in regardless unless the phone has a high IP rating like IP67. It's more a question of how long it's exposed to extreme humidity. I do not have any numbers on what it can and cannot handle. What I also do not have is a lot of users in this forum that has managed to damage their phone due to humidity. As long as you care about your phone, it will be safe.
He dunk the phone in water for 2 mins and it still work (6:45). Should you do it? NO. But if acidental drop in water or splashed then there's a good change phone can be save. Turn the phone off the put it in dry eviroment. Even my s8+ with IP rating suffered mosture damaged and it was out of warranty so for me there's no phone is safe to use in high humid environment for a long period of time, especially charging does speed up the corrosion. Cheer.
Good news duy! Not being IP rated was a compromise I was willing to take. It's even better it survives.