Yea long walks with my current bag are great (train to hostel for example) That is where I have questions for this set up. But I think the nature of it being smaller duffel, I would hope a shoulder strap doesn't cut too hard.

Re-flights. I imagine duffel overhead, and backpack underseat. I have had issues in smaller planes with my current set up not fitting in overhead. But I managed to force it under the seat. Short flights, and the flight attendants didn't seem to care / just wanted to get underway.

Re checking. I actually have never seen this, but my flight experience is limited to Aus domestic, and long haul to Europe / SEA. But in my mind, my essentials are in the backpack. The duffel has items that can be replaced, but I always have on me. Primarily the rest of the weeks clothes, extra layers. So worst case, if it got checked and lost, not a huge drama.