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We saw a person becoming part of the planet after being exposed to its atmosphere. We also know that this was not just a case of the unfortunate astronaut just being melted by the atmosphere and fertilizing the living matter already present on the planet, because his eyes and tongue did not melt: instead, they were changed in shape and size to the same monstrous proportions of the tongues and eyes that were already present on the surface.
The ending wants us to believe that when planet Remina ate the Earth, the Earth was not consumed like all the other planets; instead, it exploded, but somehow the nuclear shelter and its occupants survived unharmed, while planet Remina moved away from it and left the solar system. One of the occupants was also convinced that they would experience a miracle within a year, since their survival was already a miracle. How convenient.
What if, instead, planet Remina does not consume sapient creatures, but turns them into parts of itself, feeding their brains with false perceptions? This would explain the transmission of Kaneishi from the planet (his words weren't just misinterpreted: he was actually perceiving Remina as a paradise), while the ending would not depend from a Deus ex Machina: the people who think of themselves as survivors are actually on Remina and suffered the same fate as that astronaut: their tissues became parts of the planet, while their brains are merely perceiving that they escaped.
Alternatively, what if Remina wasn't trying to deceive anyone with Kaneishi's report? If the planet keeps its "prey" alive and conscious, providing for their every biological need, they may indeed see it as a paradise once they've had a chance to adjust. Imagine the towering black tendrils as palm trees, and Kaneishi's video feed could almost be a commercial for a tropical vacation spot.
Remina's normal "food supply" might consist of lighter elements, perhaps to produce energy via nuclear fusion. That would explain why entire star systems disappear in its wake, and why it went straight for the gas giants when it reached Sol. The occasional rocky planet is probably good for growth, but not particularly tasty; Remina gulps down Mars and the Moon like vitamin pills. A world with life, on the other hand, would be an exceptionally rare treat.
Planet Remina is Human Remina after Time Traveling
Eventually, i.e. after one year, Remina and the rest of the survivors would die off, but Remina would first be driven insane by the ordeal (especially once supplies got scarce and she was tortured through starvation and etc., might even resort to cannibalism just to survive a little longer); the safe room would crash into asteroids and other space debris, creating a core for a planetoid. Remina's insane mind would remain, however, and combined with extraterrestrial life eventually formed the basis that would one day become the planet Remina. Because of the starvation she suffered through, as well as her desire to return to normal/life, Planet!Remina went about devouring worlds, gaining mass in the process. Then the wandering planet traveled through a wormhole and after finding its way (say, telepathic connection with her past human self), Planet!Remina returned to Earth and started the whole thing again.
If that's true, then the cultists were right in trying to kill Remina. Kehehe
That doesn't explain why one of the Cultists has a similar tongue.
The Cultists Are Manifestations of Remina On the Earth
We know Remina can have its "tongues" take on humanoid shape to trick and attract prey. The cultists are its way of screwing with the earthlings, as well as trying to kill off the only people it knows might survive.
Planet Remina can Manipulate Its Own Gravitational Field
It can accelerate close to the speed of light (and possibly exceed it) without damaging itself, changing shape, or ejecting any of its mass into space. It's made of living tissue and has an atmosphere thin enough for humans to move around, yet it can force itself inside a gas giant without being crushed, and it never seems to change size despite consuming several large planets. It comes very, very close to Earth without disrupting its orbit, at least until it decides to swallow the planet. Its "tongue" manages to deflect a salvo of ICBMs without setting them off... or perhaps Remina had its gravity "ignore" them, and they simply fell back to Earth.