Ubuntu used to support static configuration of networking devices via /etc/network/interfaces file and ifupdown. Both have been deprecated in recent releases in favor of netplan, which is the default mechanism used on Ubuntu Server and Ubuntu Core to configure networking.
That said, in the case of Core, as systemd’s networkd only supports basic networking (ie. no modem support), many Ubuntu Core images include the network-manager and modem-manager snaps. On these images, control of networking is ceded entirely to NetworkManager, and netplan isn’t used for any network configuration.
It is possible to use netplan to load configuration into NetworkManager using netplan’s NetworkManager renderer, however to date, we haven’t used this type of setup much, if at all. One potential issue is that the NM renderer generates NM connection files in /run, which means they’re temporary, and thus are re-generated on every boot. Connections created via nmcli or the direct via the DBus interface are persisted in one of the writable areas of the NM snap. In theory, NM should be able to handle reading system connections from multiple directories, but I’ve never run NM configured in that way. Another potential for trouble would be that changes made to a netplan generated NM connection might not persist across reboot. So, while this may be do-able, it will take some investigation and potential changes to the network-manager snap and/or netplan.
Regarding the eduroam configuration, this sounds like a bug in NM that could be fixed.
The core snap does include wpa_supplicant, however there also is a wpa-supplicant snap which contains a fix for WoWLAN from S5. If/when we land this patch upstream, the wpa-supplicant snap may be deprecated.
Regarding the auto startup of an AP that can be used to configure a network connection if none is detected after boot, this can be quite tricky, as not all WiFi drivers support AP mode, and some drivers require new firmware to be loaded for AP mode. I also agree that trying to determine if the device is temporarily offline vs. requires network reconfiguration is easier said than done…