Well, I have no idea what the .MSI package is that you’re trying to install, so that makes it tough to comment on. This would be a case where you’d want to consult with the vendor who created the .MSI to see if it’s possible to successfully complete a silent installation or not with that package. Or do some googling to see if and how people are doing silent installations of the .MSI package in question. Of if there is a .MSU or .EXE version of it, then there could be other switches/parameters built in to the package that enable you to specify via a parameter to close the search/cortana as part of the silent installation.
I have never heard of nor have I ever seen a situation where a .MSI package returns 0/success but does not actually install. That sounds odd. Unless it’s returning a 3010, which is ERROR_SUCCESS_REBOOT_REQUIRED to complete the installation.