I understand your need to know the truth sandsad, but really, how will this change things? He's been secretly texting an OW, and according to him, this is the woman before the one he was sexually unfaithful with. Isn't it more likely that he has been sexually unfaithful with all of these women? The story he weaved about the most recent woman was such a ludicrous tale and like I always say (and did on your original thread IIRC) the lies that people tell often show them in a worse light than the truth.
I cannot imagine what this woman could say that would alter the position, but if you really feel you must hear it, bear in mind these points:
There is a relationship going on with your H and it is in her interests to corroborate whatever story he is telling you.
Texting her is not the appropriate means of communication. Phoning her from a withheld number will be more effective, although I suspect she will know it is you after the texts and will ignore your call.
It is obvious that she knows he is married with children. You will need to get into her mindset and work out how she is justifying her behaviour to herself. What script is she following? Talking to you will force her out of any ridiculous justifications she is giving for her behaviour - and she won't want to do that.
I imagine that you are trying to appeal to her sisterhood/humanitarian instincts, but if she had these in any measure, she wouldn't be doing this. She has probably had to demonise you to let herself off the hook for doing this. If she is not a parent herself, she will not be moved by the effect all this will have on the DCs.
Ultimately though, I don't see how this changes the end story. I want this to be your watershed moment, because if he is doing this with yet another woman when he was meant to be fighting for your marriage, what does that tell you?