I just want to be sure you understand that in that post you referenced, the problem was that there is a version built into the Compact 7 OS that does not match what you are trying to run, so the way to get around that is to create a text file with the extension .reg (for instance RemoveSqlCe.reg) with the below text in that file.
[-HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server Compact Edition]
Once that file is on the device in the root of the \Application folder and the device is cold booted, a different version of SqlCe should be able to be installed and used.
I'm not sure that the version you are trying to install supports Compact 7 though. I would try Service pack 2 (3.5.8080.0) if you are still having trouble.
After install Sql compact SP2 (3.5.8080.0) the problem solved.