Theory:Doctor Who prose discontinuity and plot holes/Spiral Scratch

Why does Mel not recognise Evelyn when she has met her in Intruments of Darkness and/or Thicker Then Water ?
 * Gary Russell specifically took the line that there is no single "Doctor Who Universe", but rather various parallel realities lying side by side. In fact that's the whole point of Spiral Scratch in the first place! In the climax, Mel also sees the Sixth Doctor from Real Time, the Sixth Doctor from the DWM comics etc. All these Sixth Doctors exist in different "universes". The same idea is also explicitly stated during the Big Finish Audio "Zagreus".
 * But if Spiral Scratch is the Sixth Doctor from the BBC Novels, Mel should still recognize Evelyn, having met her in the PDA Instruments of Darkness.
 * Maybe Spiral Scratch and Instruments of Darkness simply take place in two separate universes. After all, there are other BBC PDA(such as Scream of the Shalka and The Infinity Doctors) that take place in separate universes.