The way I look at this Gangway Door scenario is as follows: Let us assume that Lightoller really did order Bo'sun Nichols to collect six men and go and open a Gangway door for additional loading. This would have been around 01:05 am. Nichols would have been familiar with the men and so it probably would have taken him only a minute or two to round-up his six. Like Nichols, they would have been experienced sailors and IMO would have checked to see the position of the door they would have to open in relation to the waterline. Therefore, the scenario of 7 experienced sailors opening a gangway door that was partly submerged and then being washed over to their deaths seems highly improbable.
More than likely, they decided that it was more practical to open the higher (and further aft?) D-deck door but almost immediately decided for a combination of reasons that using that to load passengers would not be practical and likely very dangerous. So,
they closed that door again and returned to the boat deck, with the men dispersing to other tasks while Nichols went to report back to Lightoller. But by then Lightoller was down on
A-deck trying his futile tricks with Lifeboat #4 and so Nichols probably reported to someone else - either Captain Smith or First Officer Wilde - about the Gangway door and then went about other duties. Judging by reports of his sighting afterwards, he was probably in the stern section on the starboard side helping with loading of #13 and #15 initially and later elsewhere. Eventually Nichols remained on board and went down with the ship.
That would explain a number of things - why Lightoller never saw Nichols again, why no one on board Lifeboat #6 saw an open gangway door (it was already re-closed by the time #6 went past around 01:15 am), the reported sighting of Nichols near the stern section on the starboard side by Barrett etc.