Do caster out of combat options matter?

[Blatant Beast]

But, yes, I agree with your premise, OP.
The story your DM wants to tell will accommodate the party as it needs to. If "do you know this spell?" is going to be a wall, the game just grinds to an immediate halt and no one has fun when that happens.

That is a big presumption, it is also a prime example of metagaming/deus machina/formulaic style thought . If I wanted to write a novel, I would, as a DM, I am not writing a novel. Many DM's I know, approach running a game as having a script of events that would play out, without player intervention: and then the rest of the campaign is essentially the player foiling, and altering those events.

One can also be an Elden Ring style DM. If the players have made decisions that has ground the game to a halt, then the players also bear the responsibility to make decisions that renders them unstuck, if they so chose.

Finding a long forgotten Teleportation Circle formula to Moil, is it's own special reward.
Quality of life wise, it is cooler if the party can use this directly. If the party can not, then perhaps, they can find a spellcaster that would be willing to cast the spell for them.

If the party killed the only spell caster capable of casting the spell, and getting to Moil is essential, well I guess that is how the story proceeds.....some of the best games I have been involved in, have been games where the Player Characters have made choices that had less than optimal results.