cant answer your last question directly but I spent ages looking for a replacement mast. Local Mitsu dealer wanted to sell me one for $100. Contacted mitsu website and they told me the part didnt exist for the car.
however the Aerpro aftermarket mast for the Magna 96 and up is the exact fit. Almost all the car parts stores have them or can order them. Got mine from Repco for $42. http://www.aerpro.com.au/cgi-bin/list.cgi?list=antennas
The dimensions are correct but the ferrule and bottom end looked a little different. Just needed a little extra force to get the ferrule in and the jobs done.
oh, mine is a 91 Galant. Not sure what you use, but that picture of your antenna is identical to mine and the replacement.