Since this was my old gun I can answer most of the questions:
is the original frame useable (can Carter fix it?). I know that carter CS has been shakey, but with PK5 it has improved drastically.
The original frame and BBA was working just fine, I just hate BBA that is why it was changed over to the drop out back style.
The new frame can probably be fix if there is any leaky issue, there shouldn't be much of an issue though, probably just a mis-alignment of the pierce pin on top.
There is one more thing that I did disclosed to Feyd it was that someone decide to do a mod on the top piece, if you look at the photo you can see that the inside of the top piece is raw. The mod was done to prevent binding or so I was told by the previous owner, it still work just fine, I just don't like raw stuff unless I really intend for it to be raw.
Overall it is still a really good deal, the grip is Aluminum grip and those were pricy back when there was a bulk order for it.