FINAL QUESTION: In the last scene, what clues does Tennessee Williams give the audience that tells us that Blanche really is messed up?

The final scene could easily slip into melodrama, but Williams prevents that by writing only functional and mundane dialogue. The scene is packed with small-talk – the real action occurs only in the stage directions. Even Blanche's one speech is mostly meaningless, a bit more of Blanche's poetic babble for everyone to remember her by. The speech is even conscious of its own meaninglessness – she speaks of how she wants to die from eating an unwashed grape, as unheroic and meaningless a death as one can imagine, just a bit more fluff to tide her through to the end.