Unfortunately, I think you are likely correct. When a plane starts to descend, you can sense it, maybe not for first time flyers but anyone who has flown a few times can pick it up. Based on the transcript provided, it looks as though the Captain rushed to come back to the cockpit. He left around 10:28-29, presumably was back 2 minutes later. Prosecutor Robin said that at first the Captain knocked lightly and asked to be allowed back in, then when hearing no answer, he started to bang more forcefully. The transcript made no mention of the part where the Captain started out by knocking softly on the door but did say by 10:32, he was already banging on the door and shouting. Assuming he was back at the door by 10:31, it's safe to say he sensed the descend while in the bathroom and immediately rushed back.

As for the sight of the mountains, a few years ago, while on a transcontinental flight, the Captain told us that the weather was really good so he would be flying lower than usual in order to give us some close up views of the Rockies. It was breathtaking. Of course, the Captain announced it beforehand but I am sure there were some pax who didn't understand English but it didn't look like anyone freak out. It is not rare that some Captains will purposely point out interesting natural wonders or sightseeing opportunities and intentionally adjust the flight to take advantage of the views. In most cases, they would say something, sometimes they don't but it's obvious they are adjusting the flight to let you see it. Just because we seem to be descending towards the Alps will not immediately raise a red flag to me because I know planes will also do that if they are trying to evade other planes in the area. Really, the real red flag is to see the Captain being locked out of the cockpit and start banging on the door, now that would definitely be unusual. I hope that most of them didn't know what was going to happen.