You don't require to disassemble the pack to do what you require to do..
Take the 7.4 volts and build a step-down for the 5 volt supply, and a step-up for the 12 volt.
You can use with the "Simple Switcher" series from TI (formerly National Semi).
They offer full designs and many magnetic component vendors offer inductors fully characterized to be used with them.
Close it down, these batteries pack a lot of energy and can actually burst and flame if shorted.