I wouldn't put it past Mitchell to write a book, chronicling his life from Ba'al's extraction ceremony, to the alternate timeline, back to 1929, up to 1939 and the showdown with Ba'al, and then how he lived out his life. This could be left to the Mitchell family with instructions not to open it until (a date after SG-1 returns from Ba'al's extraction), 2008. It's also likely that he simply faded into history, and present-day SG-1 will never know what their counterparts endured.