Gadget
If I told her what I realy thought of the witnesses I think she'd walk away, but at least that would give me a clean break to get on with my life, but I'd be worried about what effect it would have on her.
You cannot mislead this girl as her poor mental health would implode when she finds out your real intentions re reinstatement ( that is to say - you dont want to come back)
WIthout a doubt she would suffer emotionally at the hands of her family . If she wants to run off and marry you what is it she is in love with - what she thinks she knows about you or the real you - would you be fair to her if you married with her not knowing your innermost feelings? Running off with you would likely destroy her family relationships and then when she discovers your real stance re the "truth" the effects on her could be catastrophic.
What is best for you - a marriage build on a dark secret - probably not. What is best for her - clearly most of us want her saved from the borg but is this the way to do it? - I doubt it.
Any actions you take should put her welfare (long term) over any of your needs and feelings. Its just dreadful that the "truth" is behind so many difficult situations such as yours.
The effect on her of your walking away is likely to be short lived - deceiving her into a marriage would be permanently damaging. I sincerely hope things go well for you and her - sorry but none of us has the magic answer.