Sorry, guys, I should've posted my first score sooner. I played a bit more here and there during the week, hoping to improve the score I got on the Monday stream. No such luck so far; I've still only made it to Stage 3 that one time.743,300
Chris, I saw on the Wednesday stream that you were somewhat mystified with the way your scores worked out. You were correct in thinking that it has something to do with the shuriken item in Stage 1. It's dropped by five little brown delta wings which fly in a horizontal formation on a straight up and down path. The trick is simply this - the item refills your Vital gauge. However, if you're already at full health then grabbing it rewards you with 100,000 points instead. That's a big score boost!
I've updated the 'Current Challenge Info' in the OP.