We are in a unique position now where the company is going to bet
everything on this one piece of software written by someone who has
never written production software before.
This is a huge risk.
That is why I would focus on the immediate problems (to prevent refactor-rage):
Is it an immediate problem that it doesn't run on multiple Operating Systems?...
If you burn it down and start rom scratch I can guarantee you will end up with a mess that isn’t working. Your best chance is to hire an excellent professional with a bit of a masochistic streak and pay him to improve this code. Won’t be cheap.
Do you have complete, detailed, and comprehensive set of requirements, preferably as executable tests?
I'm going to guess: No as the writer probably hasn't discovered those yet...
In such a case, there is no guarantee that burning it down and rebuilding it will be even comparable. As you've already noted, you cannot even be sure that the code acts the way ...