If cars have stopped spawning then your mod has an error in the objects.lua file.
When the game finds an error in the objects.lua it stops processing all other objects listed after it in the file.
.bak files are simply backup copies and not used by the game or by tilzed/worlded.
For your main issue you would be better off adding the extra cells to your map mod world in worlded and then creating the actual cell containing the rest of bridge, that way the game knows that everything that is being loaded is part of your mod and will properly be able to overwrite vanilla stuff as it should, it will also make any errors a lot easier to find.