It is possible to run the Mender update client in standalone or managed mode.
For a managed client is it possible to install a .mender file manually using the standalone procedure below, then carry on with automatic updates after this?
It will work as long as you intentionally not triggering update from both managed mode and standalone mode at the same time.
Please note that, managed mode already running a mender daemon which will look for update artifacts from the mender server. In standalone, you have to explicitly say that, there is an update using -install option.
Just note that one small difference in 3.0 and newer is that -install and -commit have been removed and replaced by install and commit. These worked in the 2.x versions also, so it should be easy to migrate.