The chain cant jump the sprocket on the crank because there is a bolt there to stop it. However, it can jacknife and jam. Possible but very unlikely.
If you are taking the cam cover off to check and adjust 'shim on top' then all you need to do is remove the locking cross slide.
Leave the spring loaded tensioner in place and this will go a long way to helping the cam chain not jump a tooth on the exhaust cam sprocket.
It is no guarantee though and if you are turning the crank with a spanner with one hand the other should be holding the chain down on top of the sprocket on the exhaust cam at the 12 O'clock high position to make it simple.
The spring loaded plunger will help considerably but its as each lobe passes by the highest lift point that they can be inclined to snap forward to relieve valve spring pressure upward on them. If this happens the cam will rotate, chain will go slack and its entirely likely that the chain will rotate round the exhaust sprocket by one tooth if you are lucky.
Since it actually doesnt fit through that gap (it will though) and you are un-lucky, it will jam between sprocket and head casting at the front and crack the head.
On several occassions when the chain has jumped a tooth on mine i have found it impossible to get it back into the correct position without some dis-assembly. That said: i am now 'shim under' so it all has to come out anyway.
At this stage, the Tensioner Cross Piece is removed, so the tensioner is not tensioned against the cam chain.
The tensioner is tensioned against the cam chain it is just that it is not locked and can back off.
I think a 'locking' tensioner of some sort is absolutely essential as a common thing i used to find on the road was that when pulling away and the revs fell low and it bagan to stall it would ping the tensioner cross slide away and allow the chain enough slack to jump the sprockets. Engine had very different drive characteristics for the second part of the journey compared to the first part when that happened.

AL