Earlier quoted context omitted.
By "second layer" do you mean lightning? Is that project off the ground yet? It's been a while since I paid attention.
It's very close, the only thing left to do is solve the Travelling Salesman problem in polynomial time.
Travelling salesman is about finding the shortest/cheapest path that visits all nodes. I don't see why you'd want to involve all nodes for a payment route.
If you meant "shortest path problem" instead, then it seems that route planners for road networks have already solved it reasonably well.
I suspect the challenges for lightning have more to do with keeping the map up to date.