any chance adjusting the tensioner will cure the problem?

__________________
'92 300D 2.5
'99 E300TD

No, it is not adjustable. If it has failed, replacing it will certainly help!

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Maxbumpo on the mechanic using the wrong procedure and to not have confidence in that mechanic.

Without oil pressure (not running) the tensioner will have a little play, they have a course ratchet kinda mechanism that always pushes the tensioner rail tighter against the chain as stuff wears out. And then when running, oil pressure keeps it in constant tension.
When turning the crank back and forth with a socket it will make the tensioner move in and out a little thus causing the cam to appear not to move.
Turning it backwards WILL put extra unwanted load on the ratchet mechanism, but it wont break anything or jump teeth or do any catastrophic damage.

That "mechanic" is an idiot. Go somewhere else for sure!!!!