u/ElectronicChoice592

Hi, now that the expansion is on sale I took the opportunity to buy it. My question is, if I start the expansion, is it like with DLCs where I can't return to the main map until I finish the story?

A friend told me that maybe he'd buy the game and we could play together. But I have completed the base game and both DLCs. Could I help him with the more difficult missions, or not?