Can these satellites communicate to each other and be used as data relays for communication with Earth (e.g. when a potential new spacecraft is on the far side of Mars)? What would be the approximate bandwidth?
Yes. Transmission relays via Mars Orbiters are possible and are actually used.
Curiosity for example uses orbiters to relay information faster because the orbiters have larger antennas. NASA's article of Curiosity's communications systems states,
Most often, Curiosity sends radio waves through its ultra-high frequency (UHF) antenna (about 400 Megahertz) to communicate with Earth through NASA's Mars Odyssey and Mars Reconnaissance Orbiters.
As per this article about Spirit and Opportunity, they also use UHF antennas to communicate and relay information with the 2001 Mars Odyssey orbiter, and Mars Global Surveyor (which lost contact in 2006).
Curiosity, Spirit, and Opportunity are not the only ones. Perseverance will also use relays via orbiters. This NASA article about the Communication Systems of the Perseverance Rover states that Perseverance uses its UHF antenna to communicate with orbiters. The approximate frequency the UHF antenna uses is 400 MHz.
