Yes, you can. At the main menu, select Campaign at the very top, from there, there will be 3 choices: Continue Campaign, Select Chapter, and New Campaign. Choose the second option, and any previously played chapters (regardless if you start a new campaign or not) will be accessible in any difficulty, I believe.
Chat with our AI personalities
no but killzone 3 will have campaign co-op
no
Absolutely not! The entire Killzone series is exclusively for the PlayStation 3. It is a first-party game and therefore will never be released on another companies console (I.e Xbox 360 and Wii).
You can't crawl in killzone but you can crouch though.
Killzone isn't out on the Xbox 360 so I doubt it will come out on the PC.