You are close to the game end, so I doubt you can miss much and it's also fairly easy to complete the remaining stuff..
Check my post below with the ultimate save for all endings + ways to get them, if you don't want to complete the remaining stuff.
Trust me you miss like what.. 1 hour(maximum 2) of gameplay, which doesn't even reveal you anything until you get to the point where my save files are at.
Also I wouldn't realy suggest playing through chapter 6 to anyone, who hasn't got much time.. the chapter 6 itself doesn't realy reveal anything during the playthrough, only beginning and the end of chapter 6 have some story value, so you don't miss much if you start chapter 6 and then instantly watch all endings(it may be confusing, and you may think I lie or something, but it's confusing for everyone, and having played through chapter 6 doesn't help with that at all)
I am fairly certain that I also found all the secrets after the walkthrough + I found a few more secrets than walkthrough has, but since I finished so long ago, I can't tell exactly which chapter they were in, 1 was certainly in chapter 6, but 1 or 2 may be in 1-5 chapters.
And yes, game is easy to complete without walkthrough(at least episodes 1-5 are very simple, 6 on my opinion is almost a different game and can be quite difficult to understand from time to time)
Some stuff is realy easy to miss though, I agree... on my 2nd playthrough, when I wanted to have EVERYTHING gathered, I used walkthrough quite a lot.(the 6th chapters plunging challenges were a breeze with that, even before I absorbed all stuff)
PS: they most likely used the rpg maker editor to find the secrets in this game(you can easily check all stuff you may want there)