If 150 girls in the 1st interviews passed and enter into the 2nd round of interview, and amongst which 100 will receive offers, it means that eventually 130 out of the 150 having the second interviews will get an offer. Here, I assume 40 out of 100 offer receipients will also receive an offer from SPCC, and 20 (about 50% of the total 40) will choose to accept offers and register with SPCC. Mind you SPCC is announcing results on 12/3/2012, some time earlier than DGS. So all girls enter into the 2nd interview get quite a good chance of being offered a place sooner or later.
Do you mean original certificates for supporting application? I don't think there is any need to bring the documents, otherwise, it would be clearly stated in the invitation letter.