I didn't see anything about behavior as it relates to personal conduct in the Stage III section nor did I find any source online that shows D-Wash is in Stage III of the treatment plan for that matter.
Again, personal conduct and substance abuse are two separate issues in the eyes of the league and NFLPA, unless this incident was the result of drug or alcohol abuse (i.e. Washington was drunk when he struck her). This has been a talking point for quite some time. Of course, knowing Goddell, he will use it as a talking point if it comes down to a suspension under the PCP.
If you think of the list of players that were suspended for eight or more games in the league, the punishments were for crimes such as homicide, sexual assault, or bounties. Players that received more than four games under the personal conduct policy (excluding the Bountygate players) were guys like Dante Stallworth, Pacman Jones, Tank Johnson, and Chris Henry; all of whom had multiple felonies and were well-known malcontents in the NFL. D-Wash, at this point, does not have that kind of reputation.
That being said, I find it hard to believe D-Wash will be out for more than eight games total.