If we placed and maintained a comet at Mars L1 point would we get a coma and tail that would add volatiles to the Martian atmosphere? I've also read comets create a bow shock with the solar wind, slowing it down, so would the comet's coma and tail at L1 provide Mars's atmosphere some protection from the solar wind?
The coma can grow to the size of Jupiter or even bigger than the sun. The hydrogen gas halo can be as large as 1 AU in radius! But by the time a comet gets to within 1.5 AU of the sun, ie the orbit of Mars, the size decreases and the tail increases as the solar wind gets strong enough to blow the gas and dust away from the coma.
The particles given off by the comet slow down and deflect the solar wind. So would this be enough to protect Mars from the solar wind? spacephysics.ucr.edu/index.php?content=solar_wind/sw/swq5.html