Unfortunately, the interventionist was misinformed. Unless you are on some form of protected leave, as an at-will employee, you could be terminated while you are in rehab.
This stems from the employment at-will doctrine, which is codified in California Labor Code Section 2922, and states:
"An employment, having no specified term, may be terminated at the will of either party on notice to the other. Employment for a specified term means an employment for a period greater than one month."
However, the Americans with Disabilities Act does provide protections to people who were former addicts/alcoholics, and your employer is required to provide you with a reasonable accommodation for your addiction, so it would still be worthwhile to make a complaint to the EEOC for discrimination in violation of the ADA.
You can do so using the information available online here:http://www.eeoc.gov/employees/charge.cfm
The EEOC will investigate the complaint on your behalf and either file suit on your behalf or issue you a right to sue letter so you can pursue your claim against your employer through a private attorney.
Additionally, if you have not been given a check for your paid time off, you should file a wage claim against your employer with the Department of Industrial Relations.
In addition to your PTO, you are due a day's wages per day you've been required to wait to receive your pay, for up to 30 days of penalties.
You can file a wage claim using the forms available online here:http://www.dir.ca.gov/dlse/HowToFileWageClaim.htm
Finally, you may want to pursue a cause of action for negligence against the interventionist for wrongly assuring you that you could not be fired by going to rehab. I imagine this was done to assure that you would go to rehab, but the interventionist seems to have failed to assure that your employer would actually keep your job open while you were in rehab. (This could have been done through an FMLA application for leave, which could have been done immediately after you went to rehab, since it does not need to be done in advance in emergency situations).