it's dead easy to snug the bolt and push tensioner into the right position ..and forget to tighten the tensioner bolt properly afterwards.
i ended up taking mine apart again from finished..because of this doubt , but mine was tight , so i didn't need to. that's me
Yes, 50 newtons
the top camshaft pulley would have been taken off to do the bottom chain ...this is not on a keyway ..and has to be tightened onto a taper.
watch Alan Howatts video ..that's the best there
https://www.youtube.com/watch?v=-l6-JUImJQwalan does not show it ..but i think i would be using glue "bearing retainer glue" on that taper.
a similar thing as yours happened to my sister inlaws Renault, destroyed engine after mech replaced belt.. this was about 2 months later. when i looked that one up ..the sprockets are held on with Renault's own bearing retainer glue on those.
all the best.mark