Manjaro packages Plasma-Desktop with an autostart desktop file for it and all it seems to do is waste CPU energy and resources. Killing it almost immediately brings the CPU temp and usage down and nothing seems to crash afterwards, so I'm wondering why it's there in the first place...

It is needed to convert xembed systray icons to SNI to be displayable, is there only one instance running or several? All recent Plasma styles made by KDE already use the SNI type so it is normally not needed and should be idling (it does for me, using Breeze-Dark).
Maybe you use a rather outdated theme or icon style?

Ah...it might have been the Egg-Icons set I was using...I don't see the same core temp/cpu spike coming from xembedsniproxy [single instance] now that I'm using the Uniform+ set (I only switched because two system tray icons looked too similar and I kept clicking on the wrong app), which is more recent and still just as appealing with more apps to boot....