In Alien Swarm, what do you get from leveling up

Healing seems to be the only way, but there are some methods that can help avoid infestation or - if you got infested after all - get enough healing in time:

1. Area-of-effect weapons such as the flamers or tesla cannon seem to work pretty well against those face-huggers, as do tracking weapons.
2. If you repeatedly get infested during a specific mission, consider taking a self-healing pack with you.
3. Have somebody in the group carry adrenaline. When anyone get infested use the adrenaline to allow the medic and the infested enough time to rendezvous - that way there will also be less demand for healing.

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.