Do caster out of combat options matter?

[JackPhoenix]

Which divination spells are you saying will help with that task?

Locate Object doesn't have that big a range, and that's the only relevant spell I can think of.

Divination or Contact Other Plane to ask someone who should know where to find the thing.

As a dungeon master, it's just a silly way to run games. Stop making fragile, linear plots. Start making robust, non-linear ones. Somebody up thread, claimed it takes 10 times more prep than what sees play, as if that's a hard barrier to clear. Anybody who thinks that, cannot count. Look: if I design four encounters and then demand that players go trough them in exact sequence, I've prepared one possibility. If I design four encounters and then let players go through them in any order, I've now prepared 4! = 24 possibilities. Of those, the players will experience one - 1/24 of the overall game space. Some of those may end up in players losing. That is fine. It is completely normal for a game.

That assumes there's any significant difference depending on the order you run the encounters. The encouters themselves are still the same. That doesn't sound like 24 possibilities to me.

The worldbuilding implications of "there are mages capable of casting Planeshift", "there are planar portals" and "it's possible to buy or craft a tuning fork for Planeshift" are all pretty much the same: planar travel is possible.

Not necessarily. Just because the same (general) end result is possible doesn't mean the implications are all the same in all cases. If you need a (fairly powerful) mage to cast the spell, the impact on the world is rather different from having portal(s) anyone from either side can use up. If the forks are easily available to each of those mages, the result is again rather different than if only a single archmage in the entire world knows how to access a certain plane.