Wouldn't it be easier to just have a look and see? You can always exit the level using the salts.
But yes, you can go back at any point via the CU. It depends on the level, but most will be slightly different and devoid of the original challenges - this includes bosses. In Sasha's mind, for example, you will be able to trigger all of the cube sides as they appeared when you went through the first time, but it won't all unfold like it did before.
Going through the levels later on is also advantageous as you get a little extra dialogue from the characters who own them when you come back. I was surprised at that when I first tried it.
The Lungfish path thing isn't redoable later on as far as I'm aware, since it's not really classed as a proper level. As such, you won't be able to return to it later. All minds can be revisited (hence the name, "collective unconcious") though.