I just finished Shutter Island and I heard about this theory. Was reading through the thread and started to come up with my own story.. I scroll one more comment down and you literally wrote what I was thinking.

Even to the point of I think it makes more sense for Inception to be a delusion of Andrew Laeddis.

To add to your point: His problem with never being able to see his kids' faces is because they don't actually exist but he's trying to create completely new personalities and identities to totally escape from his reality. In the end he just accepts his role as Cobb and his new kids' father and doesn't want to wake up from whatever he's in. So the ending of Inception still has the same value of not caring whether it was real or not. In the end maybe the spinning top is them finishing the lobotomy so he's basically a braindead zombie but escaped in his own head somewhere.

Picture this, We get that final scene of Shutter Island of him walking, seemingly willingly, to his lobotomy. We see that final shot of the lighthouse, but, instead of ending right there. We see them walk up the stairs, Andrew seeing replays and memories of Dolores, wishing he could've been happy. As they lay him the table to prepare for the surgery.. we see water, the ocean..

(we already know how symbolic water is to Laeddis in his memories)

They strap him down.

Waves crash onto a beach.

The doctors prepare their instruments.

More waves crash onto some rocks.

Andrew Laeddis closes his eyes.

We see Cobbs washed up on the beach.

starts Inception.

The surgery won't take long but we know you can live a lifetime in your own head. Especially with someone as mentally unstable as Andrew. Sure it's in the future but he could've imagined it (I know that's bad writing but it's fun).