Your DP wanting to help his 17 yr old is the right thing to do. But why has he rented another property to do this ?
Because the son wants his dad to himself...
Then you are setting him up to fail .
The son is NOT in a position to call the shots here. If he wants help to get clean he should do it in a way that is offered, not in a way he wants iykwim.
Why can he not come into your home, learn how a normal family lives, learn from his father expected behaviours , accepted behaviours. Yours and your DPs home so he would live by yours and your DPs rules.That's the way "normality" is surely.
Its understandable he will want some one to one with his dad, but this should be given on dads terms not the sons.
Drug takers are the most selfish of people with no regard for others and the needs of others... The first lesson he needs to learn is he does not call the shots, that would test his willingness to want to change .
You are in a disturbed and difficult family situation , you entered into this chaotic family late, but it was 4 years ago , the children were much younger then, so these behaviours have developed over time, not all of a sudden. Yes its hard work and resentment is a nasty emotion but if you can put your energies into supporting your partner and encouraging him to "man up" and stop being soft you may just be able to get somewhere, at least with the 17 yr old. Boundaries need to be set, rules adhered to and active listening learnt and practiced .
You cannot distance yourself from the grandchild either, that would b unfair and cruel... You can again here dictate that you dont want her friends around but accepting your grandchild does not mean you accept the company she chooses to keep does it.
Its hard , for sure. but life is shit sometimes. How we deal with it is what makes us the people we are, I am sure you want to do whats best for everyone. Sometimes that means making choices that are not so nice but give the right result for all