OK, before you go reporting a Close Encounter, let's see if we can set this straight.

First, just a touch of chemistry.  Baking soda or bicarbonate of soda (NaHCO₃) is a mild base that has a number of applications in baking.  Among those are to cause chemical leavening by releasing bubbles of carbon dioxide gas when combined with an acid, and changing level of acidity or basicity of a recipe.

The acidity/basicity can be important in a number of ways.  For example, cookies that are slightly basic will spread more and brown more than ones that are slightly acidic.  If you are making a chocolate cake that is too acidic, it will turn the chocolate color from dark brown to light brown to red.  That's part of the secret to a Red velvet cake.

Baking powder is baking soda with an acid added to cause the chemical leavening.  The type of acid varies with different formulations and applications, as can be found in the KitchenSavvy article mentioned above.

As discussed in Let Them Eat Cake, as well as the above KitchenSavvy article, when recipes use both baking soda and baking powder, there is usually another acidic ingredient such as molasses, brown sugar, buttermilk or sour cream that combines with the baking soda to create bubbles.

So, which of the above is it? 

Since baking powder contains baking soda, you can indeed use it to replace baking soda.  The problems is that the leftover acid that is not neutralized in your recipe will affect the flavor and may also affect the result.  A cookie that is supposed to spread during cooking likely won't, and it may not brown properly, or may end up over cooked if you try to brown it enough.  Also, the final color of the product may not be what you expect since the product is acidic.  So, point 1 above is technically correct, as far as it goes, but you may not like the result. 

If you insist on trying, though, you will need up to four times as much baking powder as the amount of baking soda that was called for in the recipe.   Just don't say I told you it was a good idea!

Point 2 is correct, since what you are actually doing when you combine baking soda with cream of tartar is making your own homemade baking powder.  What you are really doing is substituting baking powder that someone mixed up for you with baking powder you made yourself.  The cornstarch is just filler to make up 1/4 more so the total volume works out the same but isn't really necessary.

And point 3 is correct, since you can't just swap them one for one.  As the referenced site goes on to explain, if you don't have baking powder you can make your own homemade stuff by using 1/4 the volume of baking powder specified of baking soda plus 1/2 the volume of cream of tartar.  Since you need to include the cream of tartar, you are really not just substituting one for the other.

The postings referenced in points 2 and 3 pretty much agree in what they say.  The problem with the first reference is it ignores the other aspects of food chemistry that come into play when you start altering the acid/base ratios in recipes.

Hopefully, all of this helps.