I don't think you know what you are talking about, I have done several of these event through my 30 yrs in aviation. You need to look up FSIM8900.1 so you can educate your self in the area of FAR 121 certification.
Each phase of a certification have certain requirements that the operator has to comply with. Evacuation & Ditching demonstration is one of them. FAA specifically call this a demonstration, the crew needs to Demonstrate their proficiency for those events. As for BLTA the crew has already demonstrate this, 2nd part which is concurrent with the crew demonstration is technical (i.e) slides, in this case the slides needs to work. FAA have already seen the crew in action and will not focus on the crew for the future demonstrations, FAA's main objective are going forward the slides, the crew have more leeway for their performance. FAA will not move forward until the completion of this event. I have personally spoke to the FAA inspector in charge of this event.
