I've always been of the belief (Tinfoil hat conspiracy alert!) that the Chargers/Spanos fell on a knife for the NFL so that the Raiders wouldn't have the opportunity to move back to L.A. Which would have been a disaster for the Rams. I suspect the Chargers thought it would turn out better than it has.

Once the Raiders are established in Vegas it will be safe for the Chargers to move back without Mark Davis taking the NFL to court. 5 years sounds about right.
I wouldn't be at all surprised if Kroenke enterprises were somehow involved in a new Chargers stadium, possibly a joint venture with the NFL itself providing partial funding.