I wouldn't call it a "theory" per se. All I'm saying is that it's disturbingly possible.
From Hanners' perspective, Sven has those secret nerdy interests that Hannelore has revealed herself to like so much during her new friendship with Marigold. And I've said before that he was set up as a mirror to Marten since his very first appearance, and Marten's basically been Hannelore's #1 fascination since her first appearance.
From Sven's perspective, Hannelore is NOT dumber than a sack full of cabbages, is impossible to impress with celebrity or money thanks to who her parents are, and is sufficiently clueless to social norms that Sven's patented uber-flirts will fall flat on her... meaning that he won't be able to fall back into his usual womanizing self and will actually have to be a normal human being. Which I think he might actually enjoy, since he's trying to distance himself from that old persona. Plus, and it must be pointed out, Hanners is blonde.
That alone isn't humpy-dance material, but it gives good reason to think that this date would be something other than an unmitigated disaster from a social perspective. From there, the rest follows in a pretty simple chain of logic...
1 - Hannelore is likely to get nervous about going on a date with Sven, especially given the drama she can't not know it will cause with her friends.
2 - When Hannelore gets stressed, she has chosen to calm herself down with a drink before.
3 - If she and Sven are on a pretend date, and she has a drink, Sven is sort of obliged to have one too.
4 - If they do get along as I suggested above, they might have more than just one drink.
5 - When Hannelore drinks, her OCD pretty much deactivates; when Sven drinks, he makes poor decisions regarding ladies.
6 - Despite all her issues, Hannelore is not asexual. Give her an opening, take away her inhibitions, and who knows what might happen?
All in all, not hard to figure. Is it likely? Maybe not. But is it plausible within the limits of the QC Univese and plotlines? Very, very yes.