Yes, very much so.
Yes, that’s exactly what you should do. Manjaro is a curated rolling-release distribution, and it’s pretty cutting-edge, so it’s not a set-and-forget distribution. A willingness to pay attention, to maintain your system, and to engage in manual intervention where needed is expected. If you cannot or will not commit to this, then perhaps Manjaro is not for you. 
No. Just log out of Plasma completely, log in at a tty, and then issue the following commands…
sudo systemctl stop sddm
for i in ~/.config/plasma*; mv $i ~/.config/$i.bak; done
mv ~/.config/Trolltech.conf ~/.config/Trolltech.conf.bak
rm -rf ~/.cache/*
sudo systemctl start sddm
Log back in and set everything up from scratch again. I know it’s a pain in the hind side, but many of us have already had to do this a few times before when Plasma 5 went pear-shaped.
Because you are updating shared libraries — lots of them — that are in use already. This will mess up your Plasma configuration files. By running the update/upgrade from a tty, you are no longer using those shared libraries.
Furthermore, the update also includes an update to libpamac, which is the underlying library of your graphical package manager, and pamac is itself already quite unreliable. So you are upgrading your entire system with a package manager of questionable reliability which itself is receiving an update as well. That’s not just asking for trouble, that’s begging for it.