Well, I can try to answer how the a320 would react and
AFAIK, it should be the same on the a330;
First case: With A/THR and autopilot off in level flight, thrust at idle, airspeed will slowly bleed off while the plane maintains level flight/1g as you said. Reaching alpha floor AOA it will go into TOGA, speed will increase with the plane still in level flight until a few knots above Vmo/Mmo where it will start climbing in the overspeed protection, always remaining in TOGALOCK.
Second case: In open descent, again with every thing off an thrust levers at idle, you pitch down following the FD bar to maintain 250kt. If you let go off the stick it will maintain 1g in the descent you established. With no further corrections on the stick it will not maintain 250kt but only 1g. Now you increase thrust. This will increase your speed but still it will try to remain at the 1g descent untill again, you hit overspeed protection and it will pitch up.
This is writen out of memory without manuals at hand so everyone correct me if I am wrong.
The Bus, in normal law, will probably not stall, spin, overspeed or overstress itself if left unatended but it will happily fly in to the ground.