The problem with the hypothesis of gangway doors opening upon the final plunge or hitting the ocean floor is that (and I have not studied this at all) it would appear from what Thomas has told us that the only door that was found open on the wreck was this D deck gangway door.
Thomas explained his hypothesis and while it is just that, IMO his reasoning is sound. He says that of all the gangway doors in the bow section of the Titanic
's wreck, only this D-deck door on the port side was found open; I took that to mean that even the other gangway door there, around just 10 feet aft of this one, remained closed.
Thomas also said that the inner grill gate of the open port-side D-deck door was unlocked and in the 'stowed' position, something that would be necessary to gain access to the mechanism to operate the gangway door itself. I agree with Thomas' impression that this finding is strong evidence that there was human interference with the mechanism, which in turn could only have been actions of Nichols and his men in response to Lightoller's order.
I think myself it inconceivable that Nichols would have opened any gangway door, and might explain why Nichols didn't or avoided reporting back to Lightoller
As above. The order itself seems inconceivable with hindsight, but Lightoller did testify that he gave it and that Nichols left to carry it out. But as the said door was directly under the descent path of Lifeboat #6 but no one in that boat - people like Hichens, Molly Brown, Major Peuchen, Helen Candee, Fleet etc - reported seeing an open gangway door, one has to assume that it was (still) closed when they were lowered past. But since the door was found to be open when the wreck was discovered, Thomas' conjecture that Nichols and his men tried opening it, then decided against it, reclosed it but did not secure it properly is worth consideration.
I don't believe Nichols deliberately refrained from reporting back to Lightoller; IMO, the boatswain just could not locate the Second Officer (who was aft loading Lifeboat #12 at the time) when he returned from below and so reported to Captain Smith instead. Smith must have been around when Lightoller gave that gangway door order earlier and so it implied the Captain's consent to it. By the time Nichols returned, it was getting on 01:25am and very likely that he was ordered to take care of more urgent matters.
I am not aware of Titanic taking a 'dive down' at around the time that Thomas says he expected this particular D deck door to get to sea level if unlocked.
Neither Thomas nor anyone else said anything about the Titanic
taking a 'dive down' at the time. What he implied was that with the continued sinking of the bow, the sea level would have reached the outer threshold of the gangway door in question around 01:20am. If the door was still closed at the time - as we believe that it was - the water level outside
would soon have risen over and above it while the interior was still dry. Later of course, the interior in that sector also flooded, thereby equalizing the pressure on either side of the door and making it mechanically easier for it to open when the bow struck the ocean floor.
Which my above post leads us back to the witness accounts of Nichols being seen after the trip down below with his 'gang' and then not a lot else. Inexperienced fireman being told to man and row lifeboats, when arguably there was this AB gang with Nichols back from their task...
Nichols was seen by Leading Fireman Barrett just as the latter arrived on A-deck at around 01:40am; according to Barrett, the boatswain ordered him in and help with the rowing. Barrett was certainly rescued on Lifeboat #13 and seems to have done a good job; when Lifeboat #13 was inadvertently pushed under the descending Lifeboat #15 some minutes later, it was Barrett who climbed over passengers and used his pocket knife to cut away the falls.