Day night does affect what enemies you'll run into. Grunts sleep more at night, phantoms have search lights and are around more, etc. Coils, shield enemies, etc. also appear more in darker areas and at nighttime.
There will be multiple biomes. "Pacific Northwest" is the primary biome, but there will be sub-biomes, such as swampy wetlands, wartorn areas referred to as "the Deadlands" which feature environmental storytelling, hexagon caves, forerunner architecture to explore, Banished bases.
Nothing is stopping you from, for instance, grabbing a banshee is jumping to some objectives farther out in the story. You can do that. The story is structured with a general path, but there are multiple places you can go outside of the primary story beat that is present in the game, allowing you to stray from the path, rescue marines, fight Banished, explore structures, regardless of if that is what the game demands at this moment. The core story serves as an ever-present anchor that will often drop you off, so you can't jump ahead and experience primary story content, but side areas and invisible walls are not what limit you. He story is crafted to flow you through the areas in a natural way. So...
There is a linear thread still, and the story will progress in a linear fashion. Some areas will unlock and be visitable based on your progress in the story, or you may be able to find things because of what vehicles you now have access to, you can take Marines with you that you've rescued. You can't cut ahead in the story, but it's up to you how you tackle it.
There are many ways to find story elements on Zeta Halo beyond the "Golden Path." An example given was a crashed pelican that has skidded across the terrain.
Banished maintain their "heavy metal, red war paint" aesthetic from Halo Wars 2 which we will see in their structures, etc. We will see they have structures with spikes that were dropped from orbit and dug into the environment, slammed onto the ground, crushing whatever was below, which contrasts against the environment.
Yes, you can knock things off of the edge of Zeta Halo. An example given was a while playing they even managed to, by luck, knock a Wraith off of the ledge of the ring and into the abyss. It is difficult, but it can be pulled off with luck and skill combined.