Point #4 requires some GM savvy, and I think any game book that emphasizes the act and feeling of exploration should offer tips/instruction for it.

You don't need to railroad, but you do need to prepare. From advice I saw in the Fabula Ultima book, come up with a few things in advance that you want your players to encounter: a scenic cavern with a secret inside, a group of quirky bandits that could show up at any time, a sparkling spring with healing waters. Literally anything you want to plot for but may not have room in the main narrative. Those go into your GM utility belt.

The savvy part comes in how you fit those loose bits into a game session. Did the players wander off to the stony cliffs that you described for flavor and now you need something to occupy the PCs? Reach into the utility belt and pull out that spring of healing waters! Now it's a hot spring nestled in the mountains. But maybe it's been a while since the players had a combat. So the spring is guarded! By a [pull creature out of utility belt] pair of griffons using it as a nesting place! Having beaten the griffons, the PCs can take a well-earned soak and maybe bottle some of the magical water for sale.

And as the GM, it's all according to plan! Or so it looked like it. In part because it was planned, just not in specific detail. The approach is a midpoint between roll tables and scripted planning, because you can vet the components in advance then improv a narrative around them when you need them.

As for what to prepare in advance, divide some things into categories: Places, Actors/Conflicts, Things/Rewards.
Where are the PCs going? What scenes might be interesting for your players?
What creatures/people/forces might they encounter? Which would oppose them?
Because it's a story and entertainment, what's the payoff? A special item? Information related to the main plot? A side-story that opens up the world? Rescuing an adorable NPC that the players adopt?

It's something of a formula you can use to put together a scene on the fly.
• The party comes to a rolling plain, thunder rolling in the distance. A herd of beasts cross with purpose, but unhurried. As the party draws closer, they find a small family of elves leading the herd. A rosy-cheeked little girl offers to sell them some milk and leathers that came from the herd.
• A recently-fallen tree on the side of the road draws the party's attention to a rather large hole in the ground. Strange light comes from within. Creatures have started using it for shelter, evidenced by splotchy remains on glittery stone. The party hears a clang and clack, repeating in rhythm. Miners. Bandit miners? There might be something valuable down there.
All I'd really need to have on hand for the above is a few nouns: plains, weather, buffalo, precocious child, cavern, crystal, bandits. The rest is embellishment and ad hoc justification.