Well, I haven't done much scripting recently, but I'll have a go.
IN game Asteroid name is "Mary's Place"
Replace the brackets with Mary.Name = "Mary's Place" The quote marks are IMPORTANT!! If the you use apostrophes instead, it will not work.
use Symbol "xy" for player "Mary"
Hmmm. Possible, I think, but it would require quite a lot of scripting which I've never used. I don't think it's too complicated, just long. Possibly uses DrawSprite() in lots of if statements, or for loops.
Asteroid "Mary" is red
I don't think that's possible.
Color "Mary's" seedlings (e.g. red grain / yellow wings)
I don't think this is possible either.
Also, welcome!