Godzilla 2

Biollante was definitely still alive at the end of Godzilla vs. Biollante. She was floating above Earth in her latest form. Whether or not Erika actually escaped Biollante is up to viewer interpretation.

I don't know if I would go to Pumpkin head as the main source for info on monster possessions.

That aside I think the wait was more to show how effected Godzilla was by the ANEB, Godzilla stood there as Biollante roared at him then maybe he deemed he couldn't keep going and the attacks weren't doing enough(since he just blasted her down the throat and she still stood her ground) you could look at it the same way from Biollante's side that since he couldn't even manage to continue the fight she had won anyway. She also waited not only for him to walk away but even collapse into the water before she started to leave. The fact that you hear Biollante's roar and see her spores after spaceGodzilla's death makes it appear that she is very much alive even at that point in the series.

This truly has taken a turn from the orginal thread, first to Godzilla 2 then to Ghidorah's power now to Biollante.

"Or how Godzilla needed human intervention in order to beat Heisei King Ghidorah?"

That's actually not true, Godzilla was being choked out by all 3 of King Ghidorah's heads and in all seriousness would've beat him with or without the human intervention, since he then proceeded to blow off his middle head with his atomic breath. I don't see how the humans not intervening would've prevented him from doing so, it's not like the alien's control made Ghidorah stronger, or anything. He had human intervention, but he certainly didn't need it, it really had no effect on the final outcome. He would've beaten Ghidorah solo there regardless. 

As for Mecha-King Ghidorah, Godzilla did end up getting the final hit in (atomic breath) which sent them both into the ocean, so it was at best a draw. In fact, if anything, I'd say Godzilla outright won that battle, too, as the film ended with Godzilla getting up and roaring over his dead body right before the end credits.

As for this film: I'm all in favor of Ghidorah outclassing Godzilla, too, but that would mean needing a monster even tougher than Ghidorah for G3 if it does go to a third movie, since the general rule of thumb is you have to increase the scale for each sequel in a trilogy/franchise. That would be either MechaGodzilla, Destoroyah or *maybe* Biollante.

Kmanx

The human intervention happened before that. They destroyed the control panel while Ghidorah was literally using Godzilla as a trampoline and destroying the controls stunned him shortly allowing Godzilla to get up and turn the fight from there.

^This.