Can my gallbladder rupture during an attack since its only functioning 15%?

"Apples & Oranges": The cck-hida scan measures gallbladder emptying; it is used for people who do not have gallstones yet have gallbladder symptoms (biliary dykenisia). While there is a place for gallbladder surgery in this setting, this disease does not cause inflammation in the gallbladder--in fact, the gb looks normal. Therefore, you need not worry about gallbladder rupture, a rare consequence due to gb gangrene.

Very rare: It is extremely uncommon for the gallbladder to rupture. The 15% number you mention is the percent that it is able to empty when stimulated. It does indicate that your symptoms are real and that you should see a surgeon regarding treatment.