The “two accounts” issue probably happened because when you paired your second service (Garmin instead of Strava) you weren’t logged into CS with your original account at the time.
@JamesChevalier has merged people’s accounts in the past, but it is not a top priority I don’t think.
@f5ef14189c0e5cf4405f
The “Mystery Runner” issue is because unlike Strava and MapMyRun, Garmin does not share your account name and avatar with the connection CS uses, so you have to set the name manually.
There is an idea to add a means for you to set your CS avatar manually since Garmin can’t pull it, but it needs more votes.