Quote: "1) is the source code available for purchase, for the purpose of extending functionality."
From what I have seen written by Lee on the forums, I am assuming that the source code will not be available. He is only supporting what is called Version 1 with source code and I would assume the final release of version 1.09 before the "great" migration will have source code available. After version 1.09 is released, he will make a migration of code from both X10 and X9 into one version; probably version 2.0 and there probably will be no source code available.
Quote: "2) was it written in VB, c++, or .net?"
More importantly, to what executable code does X9 and X10 compile? Only Lee knows for sure. My guess is that both X9 and X10 versions first are compiled into a DBP version as I believe it was written in DBP. However, there still needs to be a compilation of DBP into executable code for the computer to understand. Since Direct X is utilized my guess would be a language that recognizes Direct X and also can compile directly into an executable file (a file with an exe extension).
Quote: "Or is there some way to interface with it like lua script."
No. DBP uses LUA through a plug-in called UNITY.