Your neighbour will not tell you anything and she will not show you her phone. You need to stop concentrating on her and give your husband your full attention - ask to see his phone and if he wont give it over then you demand to see it, he will have deleted anything suspicious so if his phone is clean then you will know he has deleted stuff. You need answers from HIM not from her.
The OW will not tell you what you want to know, and thats if she turns up.
My DP had an EA with a close friend, i too quizzed her but she didnt reveal anything, she did stray from the script a few times so i knew she was lying to my face.
You need to cut the OW off, tell her you are not friends and you dont want to have anything to do with her and tell your good for nothing husband that if he even looks in her direction again you will cut his balls off with a pair of rusty scissors.
If your marriage is to survive you need to cut the OW off, not contact from either of you and your husband needs to understand this, my DP knows if he contacts our "friend" again he is out of the door.
I get the need to know, i really do, i have been there, but they will minimise everything and you will never really know. Cutting the OW off was the best thing for me and my mental health, and you can dangle the threat that if she comes near your husband again you will tell her husband everything.