For the test it doesn't have to be a down, it can be any position. I chose to try a down as Molly now has a really secure stay in the down whereas it's not quite so good in sit. However, I tried your suggestion today, using the ball and sit and it worked really well. Molly already knew how to play catch and that she had to sit first for that, so it was just a matter of getting her used to a different cue (hand held above my head). I haven't yet tried it without the ball... @JulieT I also today worked at leaving Molly in a sit, walking away and then giving a cue for down, and it worked every time (without a towel or fabric scrap) up to about 20 feet (more than we'll need). I didn't work at dropping as she was coming towards me today, as I was trying Barbara's idea.
That sounds like good progress! Well, you've got a range of techniques there that seem to be helping, I'm absolutely sure you'll crack it!