Gentlemens, U speak here about 2 different things:
Point 1 is: Mel dispatch with 1 pack inop, downgraded to FL315 for Pax comfort and sufficent aircirculation
point 2 is: MEL does NOT apply in flight, as per FCOM and MEL preamble, U have to follow ECAM and/or QRH , unfortanly I don't have the A320 docs in front of me, don't nail me here I refer to A330.
In case of 2 Bleed faults, perf. emerg. descent to FL220 and let start APU for Bleed recovery.
The other case, that U loose the second pack, don't forget U have the Emerg. Ram Air Inlet left. According to AMM the use is limited for ALT<10.000ft and Delta P 1.006PSI due to the fact the air is going direct in the LP air distribution unit, but at least U have some fresh air for yr PAX.
The CPC goes in a Mode where the Outflowvalve will alsmost completly close, and a Delta P (DP) from 8 PSI sounds big , but the bleeed leackage from the fuselage is not so high like mentioned, so the volume of AIR with a DP of 8 PSI is enorm, enough time to get reduced by a almost closed OFV for descending on a safe ALT.
Hope this info helps a little bit, pls don't nail me on the A320, the last little bird I saw a yr ago.
rgds