A Clarkian Ethical Theory?

I was recently in a discussion with a Clarkian.  In my judgement the traditional divine command theory seems demonstratively false. I explained my reasoning as follows: 

(1) If the traditional divine command theory of ethics is true then moral values/duties are grounded alone in God's will. 

(2) Moral values/duties are not grounded alone in God's will.

(3) Therefore, the traditional divine command theory of ethics is false. 

Let's assume (1) entails:

(4) God can will any action or act to be morally blameworthy or praiseworthy.  [i.e. volunteerism]

Further, if we affirm (4), with possible world semantics, it seems we are left with an implausible conclusion:

(5) There is a possible world which God wills adultery to be praiseworthy in all possible worlds.   

(6) The actual world is a possible world. 

(7) Hence, God wills adultery to be praiseworthy in the actual world. 

Perhaps, this is a sloppy argument. But how does a Clarkian escape volunteerism? One possible way to avoid volunteerism from the traditional divine command theory is to adopt a modified divine command theory (e.g. Robert Adams); but it costs a philosophical price, it requires one to reject a robust doctrine of divine simplicity (i.e. the primacy of God's will: God wills His essence and His essence is His will; hence God's will and essence are one in the same qua divine simplicity). However, will a Clarkian deny a robust doctrine of divine simplicity to escape volunteerism?