Then maybe the downloaded packages are actually corrupted. You could check if they have plausible file sizes. IIRC pacman will ask you if you want to delete the non-matching files but I’m not entirely sure. They should end up in /var/cache/pacman/pkg.
Update only archlinux-keyring and try again.
# pacman -S archlinux-keyring
# pacman -Syu
In some cases you may need to re-populate the keyring.
# pacman-key --init
# pacman-key --populate
Adding to this: individual package updates are not supported. Always update your whole system, unless you know what you are doing.
You could set up your servers to connect to an internet reachable server via wireguard, use something like tailscale or a reverse ssh tunnel, though the latter might be a bit unstable on intermittent connection loss without any further measures.
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Fuck character limits for names. Looking at you, ABAP.
For your first point you’d probably want to investigate why your system doesn’t suspend or what exactly is going on. You could check logind.conf, specifically the HandleLidSwitch* keys. Otherwise, your lid switch should have a corresponding /dev/input/ event that you could maybe listen to or something.
I can’t offer much input on your second point. I think unplugging the audio jack should probably trigger a udev event that you could write a rule for. No idea about wireplumber though.
I’m pretty sure Solus is a Linux based OS, at least according to Wikipedia:
Solus (previously known as Evolve OS) is an independently developed operating system for the x86-64 architecture based on the Linux kernel and a choice of Budgie, GNOME, MATE or KDE Plasma as the desktop environment.
kmail is in the arch repository (in extra), the package is called kmail.