Overall, yeah, I personally thought that linearity was a bit unsatisfying. To be honest, after finishing it I personally believe that Aselia is the game that you play twice for the story (since New Game+ adds enough new content to make the whole game worth replaying), but you should only aim to finish all the routes if you really like the gameplay. If you enjoy the gameplay, then the multiple route system gives you a chance to play the game more, and there is a bonus that you get some information about the world and the characters only in certain routes. If you don't like the gameplay, then playing one route will give you enough information about the whole story to make replaying really unnecessary.
Which brings me to the next point.
There are 6 routes: Nozomi, Satsuki, Narukana, Katima, Ruputna, and Naya. I personally only finished 4 out of 6 routes, and, honestly, I literally don't see any reason to play all of them. There's literally even less difference between routes than in Aselia. Just a few lines and h-scenes here and there, and the last chapter is a bit different, but that's it. The only exception is Satsuki's route which adds some interesting stuff in the middle that you don't get in any other route (and introduces a major plot hole). So, honestly, I'd recommend to play Satsuki and then Narukana and skip everything else. If you really want to finish every route, play Nozomi before Satsuki and other routes after Narukana.
No, every other route is unlocked from the very beginning.
I personally recommend playing Satsuki before Narukana, or you may be a bit lost.
I never found one myself. And the problem is that the guides that exist kind of contradict each other. Overall, the route system is more straightforward than in Aselia, so you don't really need a walkthrough.
Basically all you need to know:
Narukana's route is the easiest to get into. There's a choice in a dream sequence in chapter 8 that only appear on your second playthrough which lets you choose between "Continue watching" or "Reach out for her". If you choose the later, you're locked into her route. That's it.
All the other routes are decided by the number of affection point you have with a heroine. You can get affection point by:
Choosing dialogue options that favor the heroine in some way.
Putting her into the the same squad as the MC and using this squad to defeat enemy squads marked with a heart. This is the biggest difference with Aselia where any enemy squad works.
Putting the heroine into the same squad as the MC and letting her get an MVP, which means to defeat the highest number of enemies in the battle. I believe, this gives you the highest number of affection points.
In the end you get into the route with the heroine that has the highest number of affection points. That's it.