I personally would only ever order hair colour that matched my own hair.
Even if you ordered darker hair and then it faded, there is no guarantee that it will fade to match your natural hair and you'll still get the brassy and red tones.

You should concentrate on ordering the matching colour and then preserving or restoring that colour.

A daily leave-in conditioner is essential I think to preserve the hair quality and colour.
Some people buy a normal liquid sunscreen and just pour some of it into the spray bottle of leave-in conditioner.
This is a good idea because the less layers of product you have in your hair, the less you feel the need to wash it.

It wouldn't matter if you got the system hair wet when you wash your hair, as long as you only applied the shampoo to your natural hair.
But I agree with Paul 100% that the less you wash your hair, the better.
If you're not using large amounts of styling products, it really isn't necessary.

A good place to find products is Sally's Hair & Beauty and they have several stores throughout Germany.

One further suggestion...
On your next order, I think you should drop the density of the hair around the sides & back by at least 10% at the side & 20% at the back, to get a really perfect blend with the density of your natural hair in those areas.