I just started working with MGTwitterEngine, but I believe I see the problem.
The API reference states that the count parameter is the number of statuses + retweets that the caller is interested in, but notes that if the include_rts parameter is not included in the query and set to one, the retweets will not be sent.
Looking at the MGTwitterEngine code, include_rts isn't set by the getUserTimelineFor functions. Since you need it to be, you could subclass MGTwitterEngine and make a new method that allows you to set include_rts, or you could just copy and paste the code below somewhere into your getUserTimelineFor function so that it is added to the list of parameters (you could add it right under the line that sets the count parameter if you wanted to)
[params setObject:[NSString stringWithFormat:@"1"] forKey:@"include_rts"];
https://dev.twitter.com/docs/api/1/get/statuses/user_timeline