can you make pipes on a national?

Yes, this is the most versatile torch on the market.  Some blowers only use a national torch, which would surprise more blowers.

Can devitrification be fixed?

Yes and no.  Not all devitrification is the same.

Many times it happens when glass is bent with a cold surface and warm inner.   This will cause devint to happen.

Another cause can be overworked glass.  This is just what it is and can not be less worked for many techniques.  To fix this one can use borax, salt or other things.  Elev8 Premier uses Borax to add sodium back to the glass, and WOW it works very well.

Sometimes it is just fucked.

Is it harder to do a gold fumed piece or a silver fumed piece?

Fuming is a fun technique that can be learned well, but is always hard.  Fuming gold and silver are pretty much the same.  You can not fume gold on the outside of glass if you do not put silver down first.  The gold will rub off.  Silver somehow actually bonds better to the glass and then the gold can bond to the silver.