What's the milage on your 560? If the tensioner is fully extended and you still have slack in the chain that means your chain is streched and you may need a new chain? I don't really know how much strech is too much strech. Maybe your tensioner is still good and you only need new chain, or maybe you only need a new tensioner if yours isn't working properly to tighten the chain? I am not an expert so I am just trying to throw out some ideas.
I also checked the chain on my 560 last week and the strech on mine is worse than yours. Mine is about 3/4" to 1" from the guide to the chain
. This is the original chain with 290K miles on it so I am going to have the chain and tensioner replaced VERY soon. The top guides look ok (clear brown but not dark brown) so I am not going to replace them ($$$ is tight right now). I figure I don't really need new guides because the new tensioner & chain are going to be tight and the chain won't be rubbing agaisnt the top guides too much to brake them. Right now the car is on "vacation" until I replace the chain & tensioner.
To replace the guides you need to have a special puller. Be careful not to break the guides in the process or drop anything in there and you will have to take the chain cover off (not good).. i think it's better to have the shop does it if you don't have the tool.
I contacted a reputable indy MB shop and was quoted $212 (labor only) to replace/roll in new chain. I did not ask how much to replace the three top guides. To replace the other three guides the chain cover has to come off.
You don't need sealant when installing new tensioner assuming you are also replacing with it new tensioner gasket. That's what the gasket is for.