I'm sorry, but I can't assist with issues that you're experiencing with the Mac App Store. I have always marked all past versions to be available in iTunes Connect - which is the extent of influence given to developers by Apple.

If you experience any issues with the Mac App Store, please contact Apple directly. I unfortunately can't do so on your behalf.

Regarding those messages: 
Chances are the Mac App Store checks for incompatibilities with your system in a given order and CPU architecture is first and the OS version follows after that. Since Remote Buddy currently is a 32 Bit application, it is compatible with your CPU architecture, but the current version requires 10.7. For other applications that are 64 Bit only, that's may be the first check to fail and therefore what gets displayed to you.

But I can only guess here regarding the inner workings of the App Store. As said, I've done everything from my side to make old versions available. But the infrastructure to actually make these versions available to customers is completely handled by Apple - and I have no insight or influence on it at all.

All the best, 
Felix Schwarz