Currently there is no convenient way for end end user who has purchased software from an ISV to install a side-loaded MSIX package machine-wide (for all users). Software is often licensed on a per-ma...
So I'm guessing the requirement is not only no managed deployments but for double-click as typing "msiexec filename" is no easier than typing "add-appxprovisionedapp filename".
In that case you could modify the file type association command on those machines for ".msix" to run that.