I was thinking the same thing about the recording-that it could have been doctored. I think Diane mentioned it as a possible defense. But I think she asked Carly if there are people who were there and can confirm that she did say those words (but they're all mobsters too!).
And I agree that this surrogate story line is strange, not well-researched. For one thing, I'm amazed at how quickly Kristina got pregnant and it seems that she's having the baby in less than in less than 9 months, but maybe it just seems that way. Maybe it really has been 9 months.