You need to be online to go into offline mode - can't go offline without a connection.
I've not had a single crash while playing the game myself, but do get a lot of stutter - seems to be caused by the game doing memory/texture management as it goes to try and keep the game inside console spec footprint something thats not needed on PC.
The crash, i'm pretty sure is a driver issue with my Pc which i will have to look into properly,as i had a load last week and thought i had removed the pffending drivers compltely.
I have definitely started Steam in offline before without previously being onlie so i don't know why it hasn't done it, i was more annoyed with the message of not being able to start offline because it couldn't connect to the network