It's likely an adult-themed content related to a Christmas story set in San Diego. But accessing or engaging with such pornographic material is inappropriate and may be illegal.
Well, one key difference is that the anime might compress or expand certain story arcs for better pacing. Also, the art style can sometimes vary a bit, but not too significantly.
Yes. The Nagatoro manga reached its end, giving fans a sense of closure. There might be some mixed feelings among readers, but the final chapters tied up loose ends.