German law prohibits you from having a white light on your gun? They don't want you to identify who you are shooting at night? Who the hell made up that law?
Oh well, that's what the SureFire wrist light is for. Even with the silly laws over there we were still busy coming up with bright ideas for you over here. At least there's a workable way around that law now.
Everyone I've seen at the local range who tries using one of the "Spec Ops" style stocks quickly learns that it smacks you in the cheek when the gun yaws in the stock.
Since you have a Mossberg and the safety is on top of the receiver, why not wait for MagPul to release their shotgun stock for the 590? That way, you'll have a little better grip angle, still be able to hit the safety, and you can use various cheek pieces and spacer inserts, if necessary, to put your face more inline with the receiver to reduce cheek slap and perceived recoil. Not that it makes a lick of difference when your life is on the line, but the appearance and features of the MagPul stock are more politically correct. Is it imperative to have a quick adjust stock and pistol grip, or just a reduced length of pull with better ergonomics?
If you require quick adjust, why not use a Mesa Tactical LEO stock and sling plate combination with a padded AR stock and AR pistol grip? Enidine makes a hydraulic recoil buffer for it, too.
I really like the GG&G sling plates on my 870, but they make the same product for 590's. I think Blackhawk even makes a sling plate for the 590. I'm not sure how much your 590 will weigh in its final configuration, but I don't find single points that comfortable with a 9+ pound gun hanging off of me. Getting smacked in the knees with it isn't much fun, either.
I like two point Blue Force Gear slings, but it's a personal preference. I like the fact that you can adjust it using the slider tab to quickly use the gun or go hands free.
Apart from that, a XS Big Dot Tritium front sight might be in order.
I like my Aimpoint Comp M3 on my 870, but a Comp M4 or Micro would work, too. I like the larger tube of the M3/M4 series, but some people like the light weight of the Micros.
Perhaps a side saddle for extra shells on the gun? I'm not sure what German law says about that. Just a thought.