Screenshots from an old favorite of mine, using the look I mentioned (though you could make the angle a little closer to almost sideways and still get away with it), yet another on my "I wanna remake that" list, lol.
As you can see, if you put invisible collision boxes at the base of walls instead of covering them completely, you'd still stop characters going through them, but allow them to go behind them. Then fade the wall out a little so you don't vanish completely (making sure you've got some artwork behind it, so when it fades out to show your character, it's not void behind there).
You could always make four layouts per level. With different angles of everything, all repositioned to match, and then any etc. characters, objects etc. only created in the first layout, and the others all told to copy them from the first, put reposition everything. But I'm not sure if that would cause things to stutter when changing angle/layout, and it would probably be more complicated than it needs to be.
If Meshes were fixed for tiling though, and could have their angle of distortion changed (so instead of coming at the screen, they'd go toward the top of it) that "might" work and let you rotate to any angle smoothly, without requiring lots of extra artwork to display backsides (use pre-distorted images which look terrible flat, but perfect when stretched by the mesh). But I've a sinking feeling that meshes wouldn't rotate correctly in construct to do that, and they'd all end up rotating locally while rotating around the player, rather than just world rotation.
You could also use a magnify effect with a gradient, so the lower area of the screen magnifies more than the top, giving a fake mode7/perspective look to the screen (from an old thread I made about it). Coupled with the already angled artwork to appear more from the side than the top, it might help add to the feeling of over the shoulder view.
You could also look into what madster is working on, for the brawler/Golden Axe style games. That might work for something like this too?