hey the half hour can only be added in case there are exactly 9 wickets down, it cannot be done in case 8 wickets are down and it cannot be done even if only 1 run remains to win.
though i am not sure if it can be done on the last day on other days it can definitely be done.
more than 60 overs can be bowled within the stipulated time, so if 60 overs have been bowled and there is still some time left in the day, match wud continue till the end of time
in the last hour of the last day, it is mandatory to bowl at least 15 overs, so if more than 75 overs have been bowled in the day when the last hour starts, the match would continue until the 15 overs have been completed in the last hour, so its possible that both 90 overs have been bowled and time is up but match still continues because of this rule.
hope that clears up the situation somewhat.