How can I get these NPCs to appear again?

Try teleporting the missing NPC to your location through console commands.

Console commands to teleport the missing NPC to the player's current location:
prid <RefID>
moveto player

prid targets an entity. Maven Black-Briar's refID is 19DD1. Carlotta Valentia's refID is 1A675.

(Another method to target instead of using prid <refID> is to open up the console and then click on an entity, this will automatically do prid <RefID> for the clicked entity. However, this method won't be useful in your situation as the follower is not near your vicinity, and can't be clicked on.)

moveto player moves that targeted entity near the vicinity of the player.

The missing NPC should now be near you after doing this. If for some weird reason, he/she is dead (could happen with Carlotta, but not with Maven, an essential NPC), use the console command resurrect 1. The missing NPC should go back to life.

If for another weird reason, the missing NPC is still not near your vicinity after doing this, he/she may have been 'disabled' from the game, making him/her not appear in the game. Type enable in the console to remedy this. If he/she appears but is already dead, type resurrect 1.

More info on console commands here: http://uesp.net/wiki/Skyrim:Console