I want to use buttercream icing to cover my 9.5 inch diameter round birthday cake.  (it will be 3 inches high) I also want to use buttercream icing to decorate my cake with shell borders, 2D tip swirl drop flowers,  roses, and leaves.

The Wilton recipe for buttercream icing is 4 cups of powdered sugar, 1 cup of vegetable shortening,  7-8 tsp of milk or water, and 1 tablespoon meringue powder.

Is it okay if I use 1 cup of butter instead of vegetable shortening when making the icing?

Also, about how many cups of icing total would I need for covering and decorating this cake?

What if I use boiled icing instead of buttercream icing? What differences would it make?

No you cannot substitute butter for the shortening in that kind of recipe. Icings made with shortening are not buttercream.  Unfortunately, they call it buttercream because few clients will buy it if they call it crisco icing.  

The reasons those recipes use shortening are: stability (holds up in heat better); shelf life (it will last a lot longer); lower change of bacteria developing; cottage food laws in most states prohibit the sell butter and/or egg based icings.

I personally never use shortening icings.  The two real buttercreams are Swiss Meringue buttercream and Italian Meringue buttercream.  They are a bit more work, but they are in my opinion worth it. 

I found the keys to successful buttercream are quality butter and cane sugar. Beet sugar doesn't make too much of a difference in SMBC, but I see a difference in the Italian because the sugar is boiled.  Beet sugar tends to crystallize into a blob when boiled.  There Is a pretty good tutorial on SMBC by Beyond

Buttercream.  

Actually in that recipe you can substitute butter... you may not need to add as much liquid though...

It won't hold up to being outside on a hot day... but yes, butter can be substituted... just make sure you don't add the "butter flavoring" that wilton tends to tell you to use in it...

It also may melt faster in your hands depending on the temperature of your hands...

If the "boiled" icing you are talking about is the cooked flour variety... I have used both butter and shortening in it... it does have a lower melting point and can also melt some if your hands are warm... you can also add powdered sugar to stiffen it up a bit more for the drop flowers...

*edited to add about less liquid*

Yes, you can definitely substitute butter for shortening (although I've never put meringue powder in mine).  Alter the ratio of butter to sugar to taste.  That's a standard buttercream here in the UK - 2 parts powdered sugar to 1 part butter by weight.  Butter will have a lower melting point than the shortening though and remember that it will have a slightly 'yellow' tinge (because butter is yellow!). The more you beat it, the paler it will become but you will never get it white.  You can colour it and flavour it as you would any other.  Bear in mind though that it will probably taste a lot different to what people are used to (I've never had a buttercream made with shortening but from what my american friends tell me, it's weird!).

Not sure what you mean by 'boiling' icing...I use a standard buttercream (above) for all cakes.  The amount you need would vary by how thick your layers were and what effect you want.  I always ay make loads and freeze the leftovers.

Yes, that's a basic American buttercream version of buttercream. The term "buttercream" refers to creaming the butter," not using only butter in an icing, so the argument about shortening vs butter with the confectioner's sugar icings is really more about the flavor. I don't like all-shortening buttercreams so I never use them, that's my preference.

I do an all-butter version of that recipe and add some meringue powder, and I also do a half and half shortening and butter version without meringue powder. There are also meringue buttercreams, which only use butter, but I've heard of some people putting shortening into those, God knows why.

Then again, you can combine confectioner's sugar buttercreams with meringue buttercreams and that works.

The terms "icing" and "frosting" are also interchangeable, one doesn't refer to butter and one to shortening.

So relax and do what you want (and what the clients prefer, that's what I do since I'm making the cake for them and not for me.) I prefer butter to shortening because it tastes better, but a lot of people prefer the sweeter taste you get with shortening. You should use whatever icings your clients prefer and what will work with the weather where you are (any kind of buttercream outside in 100 degree heat will melt even if it's all-shortening, don't kid yourselves.)