Suterare Elf-san wa Sekai de Ichiban Tsuyokute Kawaii! - Vol. 1 Ch. 0

[ShinduPK4]

If you save the world, you'll receive a special reward. You can either gain the right to live forever in the saved world of Valhalla, or return to your original world and have one wish granted!

Does this mean only one person will receive the reward? Will all the other reincarnated people get booted from the world and return to Earth? What if the reincarnated people form a party and save the world? I feel like those are rather important points that should have been clarified.

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What kind of shitty RPG is all about trial and error?

A lot of them. Even traditional RPGs like Skyrim are all about trial and error if you play on higher difficulties. There are also whole genres centered around it, think of roguelikes, roguelites, soulslikes, etc.
I do agree that it's uncommon for MMORPGs to be centered around trial and error, though.