I really think the Indoc. theory for the ending is correct. I thought the ending was ingenious, meant for the hardcore fan. All 3 games emphasize Indoctrination of whoever has contact with the Reapers and Shepard has a heck of a lot of contact with them. Just before the final scene he comes in contact with Harvenger, the most bad ass of the Reapers.
As for closure with your friends, I felt that was what Mass Effect 3 was up until the end. You find closure with everyone you had come in contact with over the last three games before taking on the final mission, the suicide mission.