The suggestion that KL will bill you for the fare difference is theoretical because there are no reports of it having happened to individual travelers. There are reports of carriers back-billing TA's because it is simple and.those are commercial relationships. No idea what the TA does when it happens.
The easier tactic and there are numerous examples worldwide is for hidden city ticket fraud to result in account closure, loss of status and cancellation of pending redemption tickets. That is because the carrier holds all the cards.
In OP's hypothetical, the result is simple because he would be skipping a segment. That, of course, results in the cancellation of the return, leaving him to purchase a single to get home at walk-up prices.
It is always possible that OP will find similar savings in two singles, but unlikely.