[Tsuwabuki]

There's some argument, but it's generally thought that WWIII occurred, despite the senshi's best attempts to stop it. Out of the ashes, several new nation states formed, one of which being the Crystal Millennium Empire, an absolute monarchy based in Japan with Neo-Queen Serenity (Usagi/Serena) as its Head of State, based in Crystal Tokyo. Serenity II (Chibiusa/Rini) is next in line for the throne in the 30th century. Senshi can die, but providing they don't have their hearts busted, they live a long time. The outward extent remains unknown and untested. At this point we know the Planetary Senshi at least last for another thousand years.

There are no timeline contradictions really amongst the different seasons. They all fit together well. What does differ is the timelines between the manga and the anime. Much of what we know about the anime timeline is conjecture based on the far more detailed manga timeline.