I reread the original post. Only one more question. You said that you backed out on the adjuster screw as far as it goes. Did you mean that the screw stopped coming out or that it came out so far (I think that it might come out completely) that further adjustment would have accomplished nothing.
You pritty much have it nailed. If the screw is our far enough, (check the screw and adjacent surfaces for burs) the cassette is in far enough on the wheel and the derailleur is not bent then ??? send the bat signal. Start checking out the derailleur exploded view for missing or misassembled parts from Shimano
Most derailleurs that I have worked on will go quite a bit farther out then the last gear.!@#$%$