Here is the "fix" I've come up with for the DLC crash issue. If you try starting directly in offline mode, I believe you with have issues with the DLC authetication and won't have access to all the content. This list below gets around this issue. Hope it helps some else, as it looks like people are still having this problem and new people are still getting it.
He is the work around I have come up with. It really helps to have dual screens and to run ME3 in windowed mode.
1) Open Origin on second screen
2) Start ME3 on main screen
3) Click the "Press any key to start" message in ME3
4) As soon as you connect to the EA servers and the system maps pops up on the right hand side...
5) Click on Origin to make it the active window, and select "Go offline" in the File menu in Origin.
6) Click back on ME3 to make it the active window and click the OK message that Origin went off line.
7) Click back on Origin and go back online (click on Go Online item in upper left side of Origin window).
8) Quickly click back to ME3 to make it the active window and click the red "Connect to EA server" button
9) As soon as you reconnect to the servers, start clicking on the Multiplayer menu item (you have to catch it between DLC authetication checks).
10) If you wait too long ME3 will probably crash and you will need to try again.
11) If you care, to quit MP correctly you will have to repeat these same steps when clicking "OK" for leaving the multiplayer mode. It probably isn't necessary as credits/experience are already stored on the server but I like to do it just to be sure.
If you haven't yet set up ME3 in windowed mode, you will need to alt-tab between programs. This is even more tricky as alt-tab is quite a bit slower switching between programs than is clicking between active windows. If you do get it, change it to windowed mode in the graphics options menu. Timing is a real issue with this "fix", take too long to reconnect to the server and the DLC will not autheticate. Take to long to get into MP and the DLC bug will crash your program again.
Hope this helps someone as it took a lot of trial and error to get the timing down.
I wonder how many ME3 MP players we have lost (stopped playing) due to the ongoing, unresolved issue....