Several funnels fell during the sinking, but the official Inquiries focused more on the 4th funnel as they tried to determine if the Titanic had broken in two, or if the 4th funnel had simply fallen which gave the illusion she had broken in two e.g.
"She broke off at the after-funnel, and when she broke off her stern end came up in the air and came down on a level keel and disappeared.
Q - Where did she break?
A - The after-funnel.
Q - What did you notice?
A - Well, the funnel seemed to cant up towards me.
Q - It seemed to fall aft?
A - Yes; it seemed to fall up this way.
Q - Was that the aftermost funnel?
A - Yes.
I believe this is why the film 'A Night to Remember' showed the last funnel falling instead of showing the ship breaking in two. They probably thought it was more practical and cost-effective to just show the 4th funnel falling, rather than upsetting and dishonouring the British shipbuilding industry by showing her breaking in two. One of the technical advisors was 4th officer Boxhall and he did not believe she had broken in two. What is interesting is that Alfred White climbed up to the very top of the 4th funnel and was still at the top when she broke in two and the funnel fell over. Incredibly, he survived and swam to a collapsible boat.
Not sure. It's possible that they didn't want to distract from the romanticized moment of the bridge going under with the band playing, but they needed something dramatic and violent for the final moments. The collapsing funnel was probably their version of the breakup in terms of cinema climax.