>>While FTM says it supports Mountain Lion, Mountain Lion is a 64bit only environment and Family Tree Maker is a 32bit only application. According to various threads around the internet most 32bit apps simply won't run and if they do become very unstable when running in a 64bit environment.<<
I replied in the other thread on this too, but note that 32 bit APPLICATIONS run just fine on Mountain Lion. Many applications, including Google Chrome, are still 32-bit. What does not run is 32-bit KERNEL EXTENSIONS (utilities & drivers), and FTM does not supply any extensions.
You are correct that 32-bit applications can't address more than 4GB of virtual memory, and it's possible that this limitation is contributing to your issue. But a blanket statement that 32-bit apps shouldn't run on a 64-bit OS is false.