Coyoteugly:
If takeoff is performed with FLX and an engine fails the remaining engine will supply sufficient pwr to continue T/O & initial climb (performance already calculated) and hence no need to select TOGA. But I think selecting TOGA is a wise thing to be added to company SOP taking into account the rare happening of such a failure.
Now to go from FLX to MCT pwr you are right about performing any of the two actions you mentioned in yr post. however, going to TOGA will re-engage the GA modes (SRS & GA TRK at least on older A320s) so I believe going to CLB then to MCT is a better choice.
Regarding the clearing of ENG OUT on FMGC, in fact it does not do any harm to a/c. this provision (CLR ENG OUT) is there to be used for cases of spurios detection of eng out by FMGC.
For the A/THR to maintain 220 kts it has to be active (engaged). If the levers are in MCT, the A/THR is not active (if two engs running)and cannot maintain speed.
But if an eng out, the A/THR can be active if lever in MCT or lower.