The article says copyright expires 70 years from the death of the artist, not from the date the work was created. I don't know who is considered the creator of the lights though. Is it different if 'the creator' is an organisation?
I'd guess that these aren't large enough differences to afford a new date but I almost feel resigned to the idea that this is exactly what will happen.
It would depend on how substantive the changes were. But most of your proposals are probably sufficient, yes.
For instance, in the UK the copyright in a film only expires as you say in the event that the film does not have a director, screenplay, authored dialogue or composed music. If it has any of them them the countdown begins at the end of the year in which the last of them dies (which is fifty years in this instance). Also worth noting that even though the film may go out of copyright in this scenario, the script/dialogue does not until seventy years after the death of the writer. Work for hire is irrelevant to that consideration.