Well, then there are some sorts of things you cannot have in your game - like magics the PCs don't fully understand, or that they cannot design to their own specifications.
PCs: Man, we screwed up in that fight, and now the high priest has let loose Mulchinato! How do we stop this fledgling god? He's way too powerful for us to beat in a straight up fight.
PC Wizard: I'll do some research, and see if there's something that'll do the trick!
DM: You spend some time pouring through old tomes, and you discover a ritual that would enable you to lock Mulchinato back in his interdimensional cage. However, it requires the death blood of an innocent to complete the ritual....
Now the PCs have a choice - use that ritual, or not. They don't know if more research will find a ritual that only calls for celery juice, or if such a thing can be created in time. They have to choose to pursue that avenue, or not.
The fictional world has rules. That means that eventually the PCs may work themselves into a corner in which their options are limited.
1- How would you feel if they continue their research to look for other options, and would you, after your last statement, keep presenting them with those options?
2- Why did you set up such a solution that removes or heavily limits player choice? Are you just wanting to kill off characters for the thrill?
3- If you are not willing to destroy the world or have a TPK, why present either in the game?
4- If at such a level to have go around to screwing with a fledgling god to begin with, and such would be a threat, are their no clerics to ask for help from a real full god so that his worshipers are not destroyed by one of his faithful followers? The DM has his deus ex machina to allow BBEG to escape for later fights to build tension, so why can't the players empower their deus ex machina to get out of DM situations they don't like?
I am still seeing the "One Ring" game here. That is fine if you told the players they were playing a "One Ring" game to begin with, but what if they find out later such and didn't want to play the game of "One Ring". When playing Final Fantasy I enjoy and expect that, as well the CRPG options are limited by the code of the game, but D&D does not have these code restrictions, so why add them? Final Fantasy CAN get away with "if you don't act or fail the world ends" because that is front loaded. Not all people want to play D&D like that.
That is what sets tabletop RPGs apart from computer RPGs in that they don't have silly arbitrary restrictions due to filesize, memory, or coding limits. Tabletop RPGs are only bound by the limits of the DM AND players imaginations.
I am pretty sure I know how you would answer a part of the first question as you previously stated about "presenting things for the players to reject", but what about the rest of it, and the other questions?