If your Alitalia flight is cancelled, you are entitled to either:

1. A full refund.
2. Any reasonable replacement routing (e.g. FCO-BOS on DL, FCO-AMS-BOS on KL/DL, etc.). If DL does not rebook you automatically, you may have to call. It helps to have something in mind to ask to be rebooked on.

I believe usually DL will send you an email, but not always.

I would sort out the return ticket before you leave the US.