Yes, that is what the board has decided. The night of the school board meeting which I listened to virtually, the board mentioned those two names and seemed to be targeting those names. I personally do not believe they even gave any other names a look. Mechanicsville makes sense as it covers the area/town as a whole. The student body will select the mascot but I've already said Mechanicsville Mustangs has a nice ring to it. I think the only other Mustangs in the state is Monticello.
3 schools in Virginia currently use Mustangs: Monticello, Eastern Montgomery, & George Mason. Two schools- Menchville and Meadowbrook - uses Monarchs. Only Liberty-Bedford uses the nickname Minutemen in Virginia.
The Marauders also is a good nickname (similar to Raider or Pirate).