I think there is a third way. I won't say it is preferable to what you are referring to but it IS another option - and where I train, where the norm does not permit uke/nage to switch roles, it is the one I use.
If nage tenses up, turns his back, etc I continue to follow (yes, continuing the attack and as if I am trying for nage's center) - but rather than actually trying to get nage's center, I follow the lead and as long as we can maintain connection, go where rightly or wrongly I'm actually being directed. If nage is contracting and spiraling me in, rather than extending and spiraling me out, this might lead me to a place where my free hand naturally gives a little atemi. If nage wanted my imbalancing to be forward with a fall over here, I won't make the course correction to match her expectation, but will imbalance where it actually is and fall where I'm actually being put - sometimes to nage's consternation.
So I try to be as accurate a mirror as possible of what nage is actually doing.
Yup, that works, too. I think your approach and mine are really variations on the same theme. At my dojo, the yudansha are more or less expected to reverse each other if the opportunity arises, but if I were practicing with a more junior person or with people I didn't know well I'd do pretty much what you describe. From uke's perspective, the important part is knowing where the gaps in nage's technique are. One might choose -- or not -- to exploit those gaps for all kinds of reasons.