Insert an explaination as to exactly how Worf is there in uniform.
Meh, didn't need one really. By "Insurrection," the explanations were getting silly. "I was in the blah blah system and..." Obviously he returned to Starfleet and the Enterprise by the time of "Nemesis." What else does anyone need to know? I don't think anyone besides DS9 fans were wondering anyway.
Worf's presence is the least of the film's problems.