Did the events of The Different Story Volume 1 occur in all timelines?
According to the drama CD that the first volume of Different Story is based on, it is in fact canon to all timelines that Mami and Kyoko met. To quote the wiki,


  The timelines diverge on the day when Homura wakes up in the hospital, so events prior to that are unaffected by her resets. 


While we don't know the exact date that Mami and Kyoko met (we do know Homura wakes up on the 16th of March), the drama CD does go into detail about the events that lead to Kyoko's father killing the rest of her family before killing himself.

Also, at the end of Episode 4 when talking to Kyubey, Kyoko refers to Mami by her first name only, and according to the wiki,


  This was considered a sign that Kyoko and Mami had known each other previously.