Even if she gets an immunity deal, she can't testify about anything her husband told her without his consent. And in most states she couldn't testify about anything Walt did absent the assault charge. Maybe some of the conspiracy to launder money, though you run into issues with attorney privilege (Saul) there.
Huh?
Skyler can testify against Walt on the charge that Walt hit Skyler. She can't testify against Walt on charges that Walt did stuff to people who aren't Skyler.
Why not?
Spousal privilege.
First, to clarify, she could testify to stuff she saw Walt do if she wanted to testify. But she couldn't be forced to testify. A person cannot be forced to testify against his or her spouse in a criminal matter.
Assuming she wanted to testify, she could not testify about anything that Walt told her. She could testify only to stuff that she has independent knowledge of.
But almost everything truly incriminating that she knows on the most serious charges, she knows only because Walt told her stuff.