At my airline ( shortly to retire the A306) for an NPA the procedure is to set Go-around altitude in the FCU whilst the FD is in ALT mode. As you approach the descent point V/S is selected and we start the descent about 0.3 nm before so that the AP has time to react. If you select MDA in the FCU you could probably descend below it if you go around and you also cannot use the AP/FD to go -around as the guidance will be in ALT( hold at MDA. We have never used Profile mode for approaches and its poor performance in the descent phase has meant that most of my colleagues have no faith in it.