No flips for you!
Except if that same forest encounter appears in any forest the players might go that is illusionism. They might choose to whether to go to Grim Woods, Ghastly Forest or the Nasty Grove but the same encounter will still be there. Reskinning that same forest encounter to different environs that the characters might decide to go to would also be illusionism.
No, it's not, unless the offerred choice is to go to one of those other places to avoid that encounter. You're still confusing prep with Illusionism because your focusing on using prep elements rather than what choice was offered.
So what does 'decided' mean in this context? If I write on my notes "this happens no matter what" that seems to be pretty firmly decided. What if I just thought about it? What if I instead though "I try to get this thing to happen"? What if I merely think "it would be cool if this thing would happen?"
You look at the choices you offer. If you offer a choice but it doesn't matter what they choose the outcome is the same, that's Illusionism. Especially if you conceal it.
The other things you mention may lead to Illusionism but are not it on their own. If having cool ideas is removing agency to you, I'm not sure there's near enough common ground to have this discussion.
Sure, there are other ways to use Force. If you're reaching to bad faith play to make a point, it's not very strong. This is because the problem is really the bad faith play and is usually cured by playing in good faith. When ypu're assuming that Force is in play (the tipping scales) then, sure, there's going to be similarities between the different flavors of Force. Illusionism is a specific example of GM Force, as is abusing mechanics to get a preferred outcome. That these look similar is no shock.And if you had not decided sneaking DCs, the number of checks required etc beforehand, can you guarantee that you spending a significant amount of time preparing that orc encounter wouldn't affect how hard you decide to make the sneaking? This is what I mean, the difference between illusionism and the GM gently tipping the scales towards the outcome they want is really flimsy.
But, that wasn't your point, originally. You said that there's little difference between Illusionism and making things up. Again, there is, especially if "making things up" isn't assumed to be code for "GM Forces their preference."