I never considered the Del Monte as a rural branch or mixed train. It was just a secondary passenger train (with steam heat) that continued to the end. By comparison the NWP Redwood was an entirely different service.

If I recall, the SP narrow gauge ended service in 1938 when operations over the line from Mina over Mt. Montgomery were discontinued. There was some service north of Laws to Chalfant or Benton until WW2 when the line north of Laws was salvaged. There were a few railfan specials, but there wasn't any formal passenger service.

Some railroads would continue to offer passage in the caboose on freight trains but I am not aware of this being a formal SP practice anywhere.

The mixed train service to Patagonia and Tombstone ended when the lines were abandoned due to the lack of freight, so it really didn't cost the railroad much more to run the combine instead of a caboose. It was politically expedient to just abandon it all together. The Tomahawk was a steam train, then a McKeen car, then back to steam, then to a 70T diesel. SP did promote it as part of their Apache Trail bus tour between Globe and Phoenix. There was substantial protest about the abandonment especially by the Indians who had free passage. SP "arranged" for Greyhound as substitute service like they did on most Oregon branches that had passenger service. I'm not sure why or when Mina was discontinued but likely it was when they lost the RPO contract to trucks.