You probably tried this, but what about running water? On a few occiaions I have poured a small amount of water between legs to help get it started.

On aim - I haven't heard of ping pong balls, but I hear cheerios float well and are good targets.

As for losing diapers vs. cleaning messes, have you considered regular underwear with a thin waterproof diaper cover over top?

- Just a few ideas. Good Luck.