The plan to thwart Mudd was very specific: they needed a ship in close range that they could "redirect" Mudd's signal to (from the captain's chair). They also needed a ship that get the signal and show up quickly (like the Klingons would).
Perhaps no friendly ships were in range, or perhaps they were thought a friendly ship wouldn't care about weird message from a "Harry Mudd", and would just ignore it. Whatever the reason, they decided the most reliable target for the signal would be Stella and her father. Obviously, they chose right, because Stella and dad showed arrived just as expected, and took Mudd away, just like they wanted.
Part of your question is why Captain Lorca would allow Mudd to simply leave.
As a Starfleet officer, he simply couldn't justify executing Mudd. And once Stella and her dad arrived to take Mudd away, it would have been very unappreciative, and potentially dangerous, to execute Mudd (since Mudd was the only reason Stella and dad came in the first place).
If you've seen the whole season:
We eventually learn that Lorca is from an alternate universe, and isn't truly a member of Starfleet, but of the Terran Empire. He definitely doesn't have moral objections to killing Mudd.
But - his entire plan depends on his ability to maintain his cover as the "in-universe" Gabriel Lorca. If he had killed Mudd or destroyed Stella's ship, he would have seriously jeopardized his cover.