i do not know why networkmanager was not working for you automatically. if you have not installed "network-extra" packages, this could be the reason. i do not have the hardware to test this.
if more people start to complain about he same thing, i know for sure.
in the live system, in which calamares runs, all drivers and firmware are installed.
no information gets transferred from the live system to your installed system.
only the packages you select in calamares get installed to your hardware (or VM, if you install it in there).
with this ISO, we are giving the power of package customization to the user.
with great power comes great responsibility!
- you are talking about a feature of networkmanager: back up the current connection setting in a way that it can be imported to all future versions of netwrokmanager. i do not know, whether this is possible to do. if not, you would need to add this feature.
- you also need to write a program, which saves that backup (of networkmanager) to the new installation. this means calamares would need to do that. you can recommend such a feature to calamares or program it yourself and do a pull request.
mhwd is doing this, but it needs the driver/firmware files in order to install them. if you neither install the firmware files nor have an internet connection, it fails.
(i am 90% certain it works that way)
i am not familiar with that.
but feel free to develop something like that (probably an extension of mhwd, which works with different network manager tools) and talk to the manjaro team how to best integrate it in manjaro.
i am pretty sure manjaro kde or manjaro xfce would integrate something like that.