It's possible that this model was never meant to be printed (or it wasn't intended for a typical FFF-type printer). The people who have said you need to edit the stl file directly appear to be correct. Once you've got it edited, you may want to make the fill denser to prevent anything odd from happening as you print.

That kinda looks like tree supports being added inside the model, likely for an overhang above that area. An easy method to prevent them from being generated is to use support blocking. In Cura, select your model, then on the left hand side in the tools, there is the "Support Blocker" tool. Select that and click on the model in an area you don't want the supports generated. You can then select the blocking block and resize/move/rotate it just like a model.. You can add more support blocking blocks by selecting the model again, selecting the "Support Blocker" tool and clicking on the model. Add blocks and try slicing the model, if you still get supports in bad spots, adjust your blocking blocks and try slicing again. Wash, rinse, repeat until you get the results you want.

I see infil between outer and inner walls. If you want to remove inner wall you need to modify your model (CAD, blender, etc). You also have support in the middle, but it should be possible to remove it in slicer.

Not sure, but that hole might be there to reduce print time.

That is Cura automatically adding organic supports inside I think, not a modelled face. You need to use support enforcers to block supports on those faces.