I think that 3.5V are the minimum voltage to allow system bootup. But it's quite outside of normal USB specs afaik. Also the TWL4030 without capacitor will probably not perform up to anything near the 200mA which bq24150 can deliver at a proper 5.0Volt to VBUS supply.
Nevertheless your results for ENUM etc are really exciting, could you try and use a selfpowered USB hub for your tests? I'd expect it should reply properly to any data sent from N900, even when the VBUS from N900 is as low as 3.5V