In order to seduce his junior assistant, a conceited inventor exchanges bodies with an attractive stranger. But when she rejects his advances and he loses control of his research, he must attempt to reverse the exchange or else face the rest of his life living someone else's.

So you think something more along the lines of:
“A conceited inventor uses new technology to exchange bodies with an attractive stranger, but loses control of his research and must attempt to reverse the exchange or else face the rest of his life living someone else’s.”
The plot doesn’t work without the clarification of why he switches once, then wants to switch back? Any ideas on how to tweak it to make it work?

I hear what you’re saying – I use the same structure again and again. But what is the alternative? By its very nature a logline needs to be set up in that fashion. Every logline on this site (if written correctly) reads like this:

When (the inciting incident happens), a (flawed protagonist) must (achieve this goal) before (whatever the stakes of failure are occur).

The optional conjunction (step 3) is included when the goal of the protagonist changes at the midpoint (for instance, in this story, the first half of the film is concerned with him trying to sleep with the girl, the second half is him trying to reverse the damage done. Or in Avatar – trick the Na’vi into moving, save the Na’vi. Or Toy Story – get rid of Buzz, get back home).

So what’s the problem with this setup grammatically?