ASSUMING several things...
1.) Conforto is going to be healthy enough to contribute and they aren't outbid for him.
2.) Yates is going to be healthy enough to be a significant contributor down the stretch.
3.) Alex doesn't go out to chase another SP.
Merrifield is probably the perfect fit. He's not the perfect platoon-mate for Harris (who isn't likely going to be in a strict platoon anyway), but he and Duvall can fake it well-enough in CF to get by as long as you have strong defenders on the corners (Acuna and Conforto/Duvall). He's not left-handed, but would still be an upgrade over Arcia/Cano until Ozzie gets back - and would still be able to contribute, particularly as a defensive replacement/pinch-runner in the postseason.
I do think Alex can probably splurge financially to make a big play for Conforto if they're happy with the medicals, but am less convinced they'll chase Merrifield or Benintendi. They no longer have Waters to use as part of the package for one/either of them, and I feel like Grissom is "untouchable" at this point. Given the struggles of every SP in Gwinnett not named Muller, he's not going anywhere either in case they want someone to come up and make a handful of starts during the second half to rest the guys currently in the rotation. That probably means the play would be to go get Conforto, keep your fingers crossed on Yates, have Duvall make a few starts in CF against the toughest lefties, and struggle through with Arcia/Cano until Ozzie's back IMO.