Yes, all this.
You obviously want to defend your son. But the woman he cheated on was a friend of your other sons fiancee. And probably hearing you going on and on about how untrue it is and what a poor little lamb derek is for being accused of cheating, when lets face it, he probably did, has likely royally pissed her off.
She also sees how quickly you can turn your back on a family member not related by blood, blindly backing up your son simply because he is your son. Your view of not getting involved and letting them get on with it, fine. But you have totally outcast this other woman and she likely realises now that you would do the same in a heartbeat to her. You seem loyal only to your sons. Thats a very clear message she has received.
She has twice criticised one of my sons to me when they have upset their brother (my son who is her fiance)
So, she was backing up her fiance? I'd say that is a good quality, not a bad one! And backing him up because your mummys boy potential cheating son had upset HER fiance. So basically you are critisizing her for standing by her fiance because you can't hear a bad word about your other son's behaviour then. Do you have a favourite son by any chance, where the sun shines out of his arse and he can do no wrong, because it actually sounds like you might.
And as for her not coming down to chat with you when she is working. That is because she is actually working. She comes down when she finishes her working day. That is normal OP. And I expect your view of her not having a 'proper job' has also royally pissed her off.
I am not surprised she has stepped back from you. She can totally see what your opinion of her is, and can also see that you would be fully prepared to stab her in the back should your son do anything out of line - in the name of loyalty to your son. Just like the other poor girl it seems.