...After some more research, I have found out that auto iso on this camera is tricky, supposedly not doable in P, A, S, or M mode (only working when used in scene mode).....
I think you should check again.
Very likely you've got that backwards.
Auto ISO should ONLY work on P, A, S or M. It should NOT work in the scene modes! That would make no sense.
Actually, in my opinion Auto ISO should only be used with M. To me it makes no sense to use it with P, A or S.
When you use Auto ISO with M you get a form of auto exposure (AE). You select the shutter speed and the aperture, the camera then selects the ISO that it thinks will make for a correct exposure. You would use this when you need to control both the depth of field (aperture) and motion blur (shutter speed).
A is Aperture Priority AE, where you select the aperture and the ISO, leaving it to the camera to choose a shutter speed. You use this when you are concerned about depth of field, but not worried about motion blur.
S is Shutter Priority AE, where you select the shutter speed and ISO, leaving it to the camera to choose an aperture. Use the when concerned about stopping (or deliberately blurring) motion, but not worried about depth of field.
There is some predictability with all the above, since the camera is only varying one parameter of the "exposure triangle". You can use Exposure Compensation to override what the camera is doing.
P is Program AE mode where you set the ISO and camera chooses both the aperture and shutter speed. Most experienced shooters don't use P very much. It lets the camera choose two variables, which makes results less predictable. If you dial in Exposure Compensation, will it effect the aperture or the shutter speed, or some of each?
If you use Auto ISO with A or S you once again are giving the camera two variables to work with and have the same concerns as P.
If you use Auto ISO with P it's even worse... you're giving the camera three variables to work with so will have even less predictability than any of the other modes.
Of course you can do what you want and maybe you'll find some uses for other modes, but personally I see no need for Auto ISO with anything other than M.
You mentioned that "f/3.5 was flashing" in your viewfinder. That's the camera warning you that your settings are such that the image will be underexposed. Apparently by a lot, since it came out completely dark. If it had instead been flashing the smallest lens aperture (such as f/16 or f/22 or whatever), that would be the camera warning you that the image will be overexposed. It will do the same with the shutter speeds, if using S exposure mode. A flashing "30" indicates underexposure, while a flashing "1/4000" (or 1/8000, if camera has that shutter speed) is telling you the image will be overexposed.