I'd let Frenzy work on the times/day mechanic the class assumes, gaining new uses from feats or class levels of Frenzied Berserker. Frenzy could be entered without being in Berserker Strength mode, and would gain the extra benefits when BS triggered.
As far as stacking FB levels for the 5 hp per level threshold...I think they should have made it 5 per CHARACTER level to begin with. Or at least cut folks with racial HD some slack... I'd let FB levels stack, but only because I agree w/ Starbuck that BS is a crappy variant, not because it has any rules basis (and by RAW, there is none, AFAIK).