Q: star scaling calculation
The average str is about half the size of the Sun. So I get 44 km.

20 km/s is equal to $6.47 \times 10^{-13}$ pc/s, but we've just established that a pc is about 44 km in your sand grain model. So the speed is $2.85\times 10^{-10}$ m/s.

Just a double check - to travel 1 pc at 20 km/s would take $1.5\times 10^{10}$s.

To travel 44 km at $2.85\times 10^{-10}$ m/s takes $1.5 \times 10^{10}$ s.

Check.

I have no explanation for you as to why it doesn't agree wth your answer or with Joshua Winn's. But I suggest you read his problem carefully, because I rather fancy he might have done something like say imagine 1 year is equal to a second or something... perhaps.