Questions (SPOILERs)

I nodded off a few times watching this... not the movie, I was just really tired... but I might be able to answer some of these.

i. The Mystic.

No reason it had to be able to go as deep as the Arkansas. As long as the Arkansas didn't go any deeper than the Mystic's rated depth all would be fine. I'm not sure, but I think the Mystic was only taken on this specific mission for the sole purpose of extracting the Russian President.

ii. Russian destroyer captain.

Was he rogue? I was under the impression that he thought he was following orders from the official chain-of-command. Yes, I know that Andropov asked him to back down, but maybe he thought Andropov was the traitor? I mean he WAS broadcasting from an American submarine and the destroyer captain had no idea of the circumstances behind that. Or, once Durov fired on the American sub, the destroyer captain realized suddenly that sh*t (i.e. escalation to a nuclear exchange) just got really real, and wasn't willing to risk a firing squad/imprisonment/death by Tomahawk for the sake of what was supposed to be an internal bloodless coup?

iii. Sub collapsing under pressure.

At this point, the sub had already been hit several times with underwater explosions, some of which likely compromised the hull integrity. Subs are very strong when they are intact, but once they suffer structural damage -- even minor -- they become very susceptible to further pressure damage.

I don't know anything about the harbour in this movie -- whether it's a real port or not -- but it was likely a deep water port, if it was a naval base intended for the movement of submarines. The fact that the Russians had mined it in such a way to prevent enemy subs from entering while allowing their own subs to exit further suggests this.

So, the depth of water might have been enough to cause further collapse to the Arkansas.

The replies you've gotten so far are from people who don't know any more than you and you're asking the question. Thus they aren't qualified to answer. They're talking out their ass and don't know themselves.

1. It was quite clear what happened to the mystic crew. They were aboard Arkansas. They docked with Arkansas aft escape trunks, got the president and themselves aboard, then cut the Mystic loose from the Arkansas. It was then destroyed by a direct hit from an RBU mortar. I don't u know what your confusion was, the film was quite clear on this point.

2. The answer as to why, is because the movie is stupid. The DSRVs had a test depth much much greater, several times greater, than any US or Russian war submarine. USS Arkansas is a Virginia class SSN. The class has an acknowledged unclassified test depth of 800+ feet. The suspected actual classified depth is somewhere in the rough ballpark of 1500-1700 feet. The DSRV Mystic had a test depth of 5000+ feet.
The reason it happened in the film is.... Hollywood bullshit.

I keep saying had, past tense, when talking about mystic because that is another film fuckup. There is no Mystic. Both Avalon and Mystic were retired in 2008.

3. The Russian captain was not on the side of Durov's military coup. He thought he was acting on legitimate orders from proper authority. He still thought so even when Captain Andropov spoke on the radio addressing his crew. But when the Russian president began speaking, he learned what was what and his loyalty was to the President, Not Durov who is now exposed as the traitor.

4. Again, more hollywood bullshit, not reality. Polyarnyy is a deep water harbor. That does not mean the water is oceanic deep beyond the crush depth of subs, it mean it can handle deep draft vessels on the surface without grounding on the bottom. In fact even outside the harbor, the Barents Sea itself is rather shallow.

5. Again, more hollywood BS, but not because of your reasoning which is just as wrong.

You are confusing modern subs with old WW2 type subs. What you see on a modern sub is the pressure hull. The ballast tanks are internal. On old WW2 subs the ballast tanks were external in what was called saddle tanks along the side. Also there was an external superstructure deck with the pressure hull below that.
Not the case with modern subs.

Notwithstanding the hollywood BS of point 4 previously, a modern sub going past crush depth is not going to show incremental crinkling and dumpling prior to failure. It will be sudden and catastrophic failure. Over in an instant.

6. Not a technical point of the film, so no comment.