How can I play different levels offline in Alien Swarm?

Yes you can. You can right-click the Alien Swarm entry in your Library, click Properties, Set Launch Options and enter +asw_show_all_singleplayer_maps 1 in the textbox.

You can also do it using the console. On the main menu, open the console with ~ and enter this command: asw_show_all_singleplayer_maps 1. You might have to enable it in the keyboard settings.

Then, if you click Practice Offline, you'll get the same screen you get in creating an online game giving you access to all games.

I have no idea why they decided to do this. The AI works on every level the same as the L4D AI works. If you leave in an online match a bot takes over your character too.

By the way, another console hack: You can try using the server browser to find a game with enough open slots so you don't have to spectate. Enter openserverbrowser in the console to use it.

Regarding your failure to join a game, I think it's just your luck. I just choose to join an online game and then the first game I pick from the list is usually in the lobby stage. If it isn't, I recommend you exit the game and search for another.