Yes, you can run this in terminal
gsettings set io.elementary.desktop.wingpanel.datetime clock-show-seconds true
if you want to disable it replace true with false:
gsettings set io.elementary.desktop.wingpanel.datetime clock-show-seconds false
Yes, elementary OS still uses X. I do not have the device you do, but I believe a good start would be to create the files you need, as instructed in the links you provided.
I do have a ~/.config/monitors.xml, so you can safely create it (nano ~/.config/monitors.xml), with the following contents:
Thank you for having this question because it allows me to be able to solve problem in command line now, because it already happens to do that with my Dell XPS 13
you can identify the resolutions of your screens with xrandr with the flag q
$ xrandr -q
Exemple with one monitor
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
I guess you want elementary to use localtime instead of UTC to avoid time differences when dualbooting elementary and Windows.
Just execute on terminal: timedatectl set-local-rtc 1 --adjust-system-clock
You can check if that's working with the command timedatectl (may need to reboot).