Can I adjust somehow the difficulty that I get? I thought that you could do it from the workers edit, but there is nothing there.
I'm not sure what you mean by adjust the difficulty. That is determined by the bitcoin network. If you mean adjust the amount of work you get, just power down your rig for part of the day.
There has been a possibility to manually set the (minimum) difficulty (UserDiff) of the shares sent to the pool. It has been replaced by (automatic) VarDiff some time ago.
This is something different than the network difficulty.
No. VarDiff will in time will optimize your difficulty for your miners to maximize your submitted shares and minimize stales.
I'm afraid the reason has nothing to do with stales. (And where there's said that submitted shares are accepted shares?)
In fact, VarDiff tries to
minimize ;-) the number of shares submitted, trying to conserve bandwidth and server load for fast miners.