You are far too close and involved to be the support she needs. That isn’t meant to rude. But you won’t be able to remove your own feelings at the expense of hers.
^This.
Also not looking to be rude, or dismissive of your undoubtedly kind intentions - but for goodness' sake, stop & have a good long internal assessment of what is really prompting you to knock on this woman's door.
If she (by which of course I mean your dad) were a stranger to you - say you read about her husband's trial in the paper & that was how you knew - would you have felt any prompt at all to go & knock on her door?
What is it about your perception of your association with your dad, & knowledge of his crimes, that makes you want to reach out to this woman? What do you feel may have prompted your desire to tell her no one blames her for any of this and I hope she is okay?
How much of that statement is actually more to do with your own psychology, i.e. what must be some complicated & difficult feelings around your own mother?
Supposing this current wife finds your attempts at contact intrusive, or rude, or weird? And apart from that consideration - what about your own mental health? Having survived your dad once, why would you embroil yourself in what could be a volatile communication with his current wife? What are you really looking to achieve here, & why do you feel it is appropriate for you to offer support, or reassurance of no blame attaching to her?
How do you even know that no blame attaches to her? You have absolutely no way of knowing if she was complicit, or enabling, or ignorant of what your dad was up to.
It's maybe also worth reflecting on the fact that you perceive the current wife as she looks like a defeated woman, eyes to the floor, head down kind of behaviour, I feel like I need to tell her that I understand why she's still there with him and that shes not trapped -
but where is your own mother in all this?
You barely mention her in your initial post. Did you own mother escape the trap successfully? - is she living happily now, or still scarred by her marriage to your dad?
I am concerned that you are projecting in an attempt to settle some old wrongs still niggling at your own sense of how things should have been for you as a child.
Your instinct to help this current wife is commendable ... but please don't jump in without considering your own fallibility, the fact that YOU TOO were a victim of your dad (a child who witnesses abuse is a victim of abuse), or how much you may be projecting here.
My suspicion is that some buried part of your subconcious is reaching out to try & make things right for the child that you were & the wife that your mum used to be. It's likely that any communication with - let alone if that built up into a relationship, however tenuous - with this current wife is going to be extremely triggering for you.
I think you should congratulate yourself on your kind instincts, but let this one go unless you can access some form of counselling to unpick the deeper motivations underlying your instinct to comfort, protect, or 'rescue' the current wife - & how that relates to any subconcious narrative around your relationship with your own mother.
This must be horribly upsetting for you OP.
Congratulations on surviving your dad - please don't go opening cans of worms that might bite you unless you have ample professional support to help you manage the emotional fallout. 