Dude, you have the same exact boat I do! Any pics?
About the impeller, where I found most, if not all, of my impeller pieces when that happened to me was on the INLET side of the impeller. Take off the hose from the impeller to the tranny cooler and I bet you'll find chunks in there. Also re-check the four passages in the impeller housing again before you put it back together. I mean I'd take the impeller right out of the housing again and make sure everything's out of it. I lost every vane in mine last fall on my last run of the year. I cleaned out all the chunks on the inlet side. I also checked the outlet side to the t-stat and found nothing. Runs at normal temp now, but I did install this for piece of mind next time just so I know all the pieces will be accounted for. http://www.wakeworld.com/MB/Discus/messages/3183/69621.html?1054821358
As far as backflushing, I'm not sure, but I would think you'd take the 1" hoses off the exh manifolds and stick a garden hose in them. If someone knows for sure hopefully they'll post instructions. Mine is fine without having done that, though. Let us know what you find!