The EP Study is the study of the electrical system of the heart. The doctor looks for the electrical tract that the wrong electrical impulse is traveling down and then he ablates that tract. That's about the easiest way for me to explain it. One thing that you have to realize is that if the electrical tract is not 'active' on the day of the ablation, he will not be able to find the tract and ablate it. (In other words, this may be a day your daughter would not be having an arrhythmia problem if she were at home) Sometimes only a partial ablation takes place and the SVT can return so more ablations may have to take place before the problem is completely gone. Inderal can cause drops in BP; if you are using a home monitoring system, they are not always reliable; it's much more important to know if she is having symptoms as a fast heart rate also lowers blood pressure because of the amount of blood being sent out to the body.
Propranolol hydrochloride (generic for Inderal and other trade names) is a beta blocker and as such has as one of its main attributes, lowering the blood pressure. The number you show is too low, I believe, and you should discuss this with the doctor.
I don't know about EP studies relative to ablation, but it seems from what I know about it that it could be a great first step in determining what has to be ablated.
My limited understanding is that finding the unwanted signals and ablation can/do take place during an integrated procedure. This may or may not be proceeded by an EP study, I don't know.