In The Doctor, the Widow and the Wardrobe she saves her husband, but with that she modifies a fixed point in time, how is this possible? Is it a slight/tolerable nuance of a Christmas Special?
I think you misinterpreted the ending of the episode: she did save her husband, but she didn't change anything.
She had always saved her husband.
When her husband dies in the opening part of the show, we see him flying his plane back from a mission when he runs into a bright light and disappears. She gets a message from the RAF that her husband's plane went down and he has died, but they never found him. They didn't recover his body or his plane, he's merely missing and presumed dead.
What had actually happened is that the "widow", while piloting the tree-people's ship, had also pulled her husband into the future with her. The reason his plane "disappeared" was because he had time traveled forward (it's not really clear how far) and ended up at home on Christmas.