I mean, come on man, it says it is single player on the Steam page. I don't care if you refunded it or not, but I would like to know...
Did you even read the "About This Game" section?
Also, the dev stated that there will not be multiplayer... ever.
If you look around, on the Discord server, SteelRaven's twitter, and the Steam Greenlight page, you would be able to find out that some of your suggestions have already been considered:
1. There obviously will be more weapons, and Beta 6 will have a plane. You can see them in the Steam Greenlight trailer. However, he has stated that weapons will not have customization(Discord FAQ).
2. He will not be adding multiplayer in any form.
4. There will be a campaign.
5. AI is always being improved in some way.
He did say he would add a map editor and Steam Workshop support.
He won't. First, what beta testers? And if you are referring to those who played while it was in beta in itch.io, then it still doesn't make sense.
One, there's no way to find out who those beta testers were.
Two, the game will still be in beta when it goes on steam.
Also, the price range is between 12-15$ USD. - Steam Greenlight page and Discord server.