I don’t know how or why droids feel pain and emotion in the SW Universe; but speaking from a perspective of filmmaking, I am glad that they do. The characters of the droids are much richer because of it. However, it has to be handled realisticly.

Like to add to this and say that Droid pain is something I prefer Star Wars never explains. I like having it be one of those things that fans can ponder about since within the story it doesn’t need any more explanation other than “C-3PO, just like any human character, can and will be tortured if he upsets his new master.”

As for the shifts in tone within the film, I don’t think its that big a deal. The whole first act of Jedi is a bit bumpy in its execution, but I never found the tone to be an issue, at least not for me that is.