These are awesome questions! Here's a couple answers:
1) We're building our full size airship to fly at 60 mph, which increases the usability in inclement weather.
2) Joe answered this in another comment, but our airships use pretty small quantities of helium compared to the bigger ones, so it's not a major operational cost.
3) For sure this is an issue, but the bigger the blimp is, the more bullet holes you'd have to fill it with to prevent it from reaching its final destination. The german zeppelins of world war I were really hard to shoot down. See 5:47 of the below video.