How many times can Phoenix oath be used on a single piece? After the initial usage of the warranty do they need to repurchase the phoenix oath?

Over and over again.  They have to buy a new Phoenix Oath at the original retail price for it to work. For instance

The original price paid $100, + they paid $8 for the oath at 8%.

They get a new piece for $50 and if they want to cover the original piece it is going to be 8% of the original price, so they would pay $8 for the warranty.  This would cover $100.  So they pay $58 to get a new piece replaced and to have a new warranty to cover that same piece.  They can do this over and over and over.  If the piece is no longer around, they can get another one that is the same original purchase price.

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.