Visible crew/equipment: While the fake band members begin to practice before the mayor shows up, in Andy's closeup right before he tries to stop Phil's flamboyant playing, the reflection of filming equipment is visible on the surface of Andy's tuba.
Visible crew/equipment: At the start, when Andy and the rest of the band are at Floyd's, we can see the black tape mark on the floor behind the barber's chair where Floyd's standing. Watch Floyd walk away from the chair for a moment, and when he goes back he looks down twice to make sure he's standing at his mark.
Answer: It was a joke. They were supposed to be dumb hillbillies.
Both. According to later interviews with both Andy and the Dillards band (the real name of the band that played the parts of the Darling children, of which Charlene (Maggie Peterson - Mancuso was also a member) the reason the Darling boys never had any lines was because they would've had to have been paid if they spoke. In return for appearing without lines or pay, Andy promised to get as much of their music onto the show as possible which, he did. This obviously gave them a lot of exposure.