Changing the hostname should be in System > Status > System, but that is unavailable to bbb.
Yes you cannot rename the user. Well you can but its pointless, it still needs to be root level and some things may break so you better don't.
Where do you get the idea to use commas? I has always been dots, anything regarding IP numbers has always been dots, at least for IPv4.
Do not use the Network tab, use the (Advanced) one. The "Simple" one should be fixed in a future release.
Note that for bbb you don't have this option, it is more complicated so its best to open a support ticket or come to the telegram support group and ask there.
The '[Graph QL] Network settings could not be obtained' error is very annoying, it pops up every time i select the SYSTEM tab!
It will not save static IP address in normal tab, it will in advanced.
Hopefully the bug will be fixed soon.