You are right that the Democrats can run another stooge during the special election just to muddy things up.
People might vote for the special candidate thinking that they voted for the permanent candidate.
Democrats can use the special candidate to say awful things about Brat (who might not even run in the special election) just to taint him with campaign monies not tied to the general election. In effect, Brat might be running against two Democrats.
If Cantor does this, he should fund Brat's campaign for the special election, too. Let Brat run in both elections and get seated immediately.
Best is for Cantor to honor his commitment, stop being a spoiled brat himself, and finish out his term.
-PJ
Identify the Brat ...