First, you are not addressing Apple on these Communities, we are fellow users with the space provided by Apple.
Second, the Mini will take 16 GB memory, but be sure to buy quality, Mac-cerified ram from Mac specialists like OWC, http://www.macsales.com or Crucial, http://www.crucial.com they have kits that will max out the memory.
Third, failure to release memory after closure is in almost all cases a problem with the way the software is written and not the hardware. This has been a long-standing issue with software developers, writing code that does not release the memory used when the software closes. Often it is necessarey to restart a computer when this occurs so the memory is released. Do you see this happening with all software you use or only certain programs?
This problem is consistent across multiple applications. This is a classical OS issue because the OS should be managing resources and not allowing processes to consume all of the memory. Leaving a user with 5MB of memory to work with is ridiculous. Key processes that cause the problem are background tasks, like CalendarSync, AddressBookManager, SyncServiceAgent, SystemUIServer, then apple mail consumes a lot. the problem is progressively worse when Time Machine starts. This needs a fix from Apple and fast. There are probably thousands of users who are having this problem but think its a program that is causing it and not the OS. In other words, even if it was a program problem the OS should prevent it, and in this case the problem is program agnostic.