Oh, I'm not doubting that the concept of Lady Heather dying came up in the writing room, but from what I have heard, in terms of actually putting pen to paper, Lady Heather was never going to die once the scripting was in progress.
I do imagine that WP would veto Lady Heather's death if it came up, as would I. I see no point in killing her off especially when they couldn't possibly give it the attention or time it deserved for aftermath.
But in terms of what physically made it onto any given draft of the episode, I believe that story has been made up. And the WP thing... again, I can see it, but I've heard nothing of the sort from any official sources.
I don't want to come off as a know-it-all, that's not my intention, I just think that a lot was made of what this episode meant in conjunction with "another" (lol) shipper group, and (I hope) both sides came off happy with the product.
But given the entire theme of the episode with the granddaughter, I can't see Lady Heather dying being any more than an idea thrown around the writing room. Because if she did die, what would be the point in trying to look up the grandchild?