Obviously this scenario would also involve a time pressure, otherwise the choice would not be a choice.
Exactly. This is it, 100%. There is no choice unless there is a time pressure. And if the time pressure is "you have three days" then it is also not a choice, because you can't afford five days of travel.
And this also ties right back into the question. Which road to take isn't a choice, it isn't a challenge, unless there is a time pressure of let's say 6 days for this example. So, do you honestly try and have a ticking clock for every single overland travel, every single dungeon crawl, every single city exploration... how are your players supposed to take the time to actually explore and experiment if they must be constantly rushing?
Possibly where your thinking is going wrong here is in the phrase 'begin the adventure'. I'm not designing wilderness travel to waste time until we get to the adventure. I'm designing an adventure, part (or, perhaps, all) of which is set in the wilderness.
If going through the Forest of Random Encounters is the adventure you want, why are you presenting it like a choice? Just tell the players.
"You could take the high road, but that will take 5 days, and in 4 days it will be too late, so you turn your gaze to the dangers of the Forest of Random Encounters it is the only way"
Or, just say "hey guys, I have a cool forest adventure before we get to the temple, let's take the forest road, it'll be fun."
But now, how do you make the forest exploration interesting?
My time pressures are never an illusion, they are they to put choices in the game. This whole conversation is very strange. You seem to be imagining ways that someone could design a clock so that it didn't matter. But obviously I wouldn't design my clocks that way. That's the whole point. And of course I could let you arrive early. But the camp where the cultists are is fortified, and you'd have to sneak or fight their way in. And defeating the evil mages would be more difficult without the Magic Item of Plot Device. But if you get the magic item of plot device, you will either arrive just on time (in which case you almost certainly win and feel like heroes) or you will arrive late. Because the detour will be designed to potentially slow you down if you make the wrong choices.
But, unless you totally screwed up the detour, you have the Magic Item of Plot Device and so still have a chance to save the day by using it to banish the summoned whatever. Or, if you show up late and failed to get the magic item, then the whatever is rampaging and you're too weak to stop it. So you either get killed or run away in shame having failed miserably. Which is fine.
I'm sorry, why does the Magic Item of Plot Device mean I don't need to sneak or fight my way into the fortified base? I still need to do that, right? So, we go and get the Plot Device, and we arrive at the fortified camp with an hour until the ritual... and since it takes longer to get through the defenses then we lose because we don't have the time to get through.
Or we show up without the Plot Device item, and we have plenty of time to sneak into the compound, but then despite the ritual being potentially days away from completion "
, then the whatever is rampaging and you're too weak to stop it." and we fail anyways. So, our only way to succeed is to take the detour... then why give us a choice? Do we even know that there is a summoned monster protecting the ritual that we can't possibly defeat without the McGuffin? What is your goal as a DM giving players false choices that lead to them being defeated if they don't follow your plan.
What you have actually done here is set up a scenario where if the party deviates from your plan, they lose. They need to take the detour, and they need to do steps three four and five, and then they will arrive in time to sneak in, beat the unbeatable monster and save the day. But if they don't take the detour, they lose. And if they get bogged down on steps 3 and 4 then they lose. And the players likely know none of this information.
This is all a massive problem with the design of your adventure, there aren't multiple routes to success, there is a single route, and you might as well just tell the players.