No, life-after-death cannot be an argument here - note that in all those things we never see anything about an actual after-life. Even when Harry wonders about Dumbledore's presence in that station scene in book seven, Dumbledore says that it may all well just be Harry's imagination, and it being his imagination doesn't make everything that was discussed wrong. (Edit: Now if resurrection was discussed...it could've been a huge talking point. But even with Voldemort its not resurrection a la Christ, as he never truly died till the end.)

Thing about that scene in book seven is, everything that Dumbledore said was something Harry could have worked out with the knowledge he had, so it well might have been his imagination after all. It keeps with Rowling's attempt at avoiding all the philosophical and religious trappings that come with fiction like this. After all, this is not an Arda or Narnia where a god, super-natural beings and faith are built into the story.

Edit: She still skirts a fine line with this, though.

 

The Bible is rife with warnings against magic and witches, so I don't think the wizarding world would be favorably disposed towards Christianity.

http://www.biblegateway.com/passage/?search=Exodus 22:18&version=NIV

http://www.biblegateway.com/passage/?search=2 Chronicles+33:6&version=NIV

http://www.biblegateway.com/passage/?search=Deuteronomy 18:10-12&version=NIV