After filling a lawsuit against a mammoth corporation, an ambitious environmentalist learns that the man representing them is the same guy with whom she had once _______

Would you consider none of the above, and go for the opposite? - a disastrous one night stand... which could create more tension that then blossoms into real romance?

the second scenario won't go,
If they had the most honest relationship and still won't see each other to the point that meeting him in such a role acts as your inciting event then they had some trouble which they couldn't possibly ignore (otherwise they'd be together still)
and once the relationship is of the hook what's stopping her from going full throttle?
Also If she really is ambitious, just a relationship history won't do
I like the third one, just don't reveal the conversation all at once, but bit by bit as the plot progresses...
good luck!