Good questions.....
Im going to start by answering your questions in more of a floor trader/directional traders method of looking at Butterflys and keep it simple.Lets assume that you do not hedge out the Delta of your chosen butterfly and keep it a straight directional bet...Of course the price of butterfly will be affected changes in implied volatility,and you should be aware of it.
Yes,the butterfly selected should line up with your forecast on the underlying.Up,down or sideways...We are going to ignore Theta(time decay) vs Delta for now and agree that you need a correct opinion of the underlying.
Like any other bet,you should know your risk reward.The beauty of a butterfly is it is clearly defined.Perhaps you pay 1 for a 10 pt wide fly.You know that the most you can lose is 1,and you can make 9 if the stock goes out at your short strike.Now,keep in mind,if you are trading a butterfly,you do have a delta and it changes as the underlying moves.If a stock is trading at 100,and you have a bullish 100/110/120 Fly on,if the stock gaps on takeover news to 130,you will lose money,and your butterfly with positive delta will now become a bearish bet as your "sweet spot",i.e your short strike is now 20 points beneath 130...Essentially,if you are betting on direction with butterflys(no hedging),you are more or less predicting a range...If it moves way outside that range,you will lose money..
You should have a decent understanding of verticals to intuitively understand flys
Let me know if you digested most of this...
If you did,pick a stock you like,tell me your thoughts on it and come up with a fly and we can go over it and discuss the basics