Original poster: You are perhaps in danger of overcomplicating your thoughts during what is essentially a straight forward type rating. Look at it in practical terms, a fuel leak is usually represented by an imbalance and the QRH is Boeings most condensed method to address this issue. I have never come across any company procedure to check fuel used prior to crossfeed without being directed the NNC or some strange anomaly.
Regarding taking off with APU bleed supply the packs as opposed to engine bleeds, again this is a relatively rare and the performance tools you use to calculate the N1 should have the option for " Bleeds OFF" to give the appropriate target N1.
The APU bleed cannot supply the same volume of air as both engine bleeds.