I can't speak for the devs, but I really doubt .psd support will ever be implemented. First off, it would be rather complex to code, I'd imagine. Second, .psd's can have vector layers, text layers, masks of varying types, adjustment layers, effects, and all manner of non-graphical data in them... so yeah, that just adds to the complexity. Animated .gifs or .apng's would probably be more likely, in the future. (I wouldn't mind .apng support, personally
As for importing mirrored images, just import all of your frames for one direction, select all the frames, copy them, paste them in the other direction, and flip them all in the image editor.
And my own personal method for making animations is by using Photoshop, and setting everything up in layers... just like you, I'd imagine... but I also add a pixel to each frame to register the hotspot so it's easy to line up after importing to Construct. It's just a matter of clicking the pink pixel on each frame (or whatever color). Then it's just a matter of a quick pencil tool on each frame to cover it up. It's not as time consuming as it sounds.