1) normal loan and cheap loan both can either be discounted using pretax debt cost or risk free.
2) this is same for calculating AF of tax shield calculation and interest saved and tax shiled foregone calculation. What i mean is if used pretax, use pretax rate AF and if used Rf, use Rf annuity factor.
3) can i mix the discount rate? eg use pretax loan rate for normal loan and rf for subsidised loan or vice versa.