Have you tried converting the file from within Fireworks, using File > Save As... and choosing Photoshop PSD? It seems to work better than simply opening up the Fireworks PNG directly in Photoshop. The slice names aren't immediately visible in Photoshop, but if you choose the Slice Select Tool and double-click on a slice, the Slice Options dialog appears and reveals the original slice name.
For quickly resizing an entire document in Fireworks, I'd try Modify > Canvas > Image Size. It should work especially well with vectors and handles slices, too. For bitmaps, keep in mind that Symbols can maintain the original resolution of an object much like Smart Objects in Photoshop. So if you knew you were creating something to be scaled up to 200%, you could try defining any bitmaps as Symbols and scaling them down 50% on the canvas. Then when you resized to 200%, they'd be rendered at their original resolution.