Hi Alex/Ardillamente.
Welcome to our Mattress Forum! 
You’ve had quite the ordeal with the VOCs and off-gassing you experienced with the Serta memory foam mattress. I’m glad that you were eventually able to get Mattress Firm to honor their return policy, and quickly find a new mattress that works for you. You’ve certainly made a good call going natural with organic cover and working on removing the trapped and lingering VOCs in your room and carpet.
To address one of your points, Serta does use CertiPUR certified foam – CertiPUR does have high standards for testing (see post #2 here for more information about some of the more reliable “safety” certifications, including more in-depth information about CertiPUR).
This is not to minimize what you are experiencing, some people may be quite sensitive to some materials, and some others have certain medical issues such as MCS (multiple chemical sensitivities) or may be sensitive to some materials that the large majority of people would be fine with that are really impossible to predict … I would suspect you’re sensitive, as you’re still experiencing discomfort even though the mattress is a long time gone from your home and you took massive action to remove VOCs from the room.
Marshmallowforme offered some great tips to help.
You already have fans circulating with the windows open, and you mentioned you are using a Moso charcoal activated carbon air filter to help absorb any lingering VOCs. You’ve also already taken the proactive step of cleaning the furniture and bedding. Some people use high-performance air purifiers along with heating the room to encourage the VOCs to evaporate (heat increases VOCs evaporation rate), circulate, and to be “absorbed” by the purifiers.
I would certainly keep doing what you’re doing!
Phoenix