It sounds like you are talking about the older M&P style slide release on the full size. They have a newer extended one that is easier to use that is on all M&P's now. As for the 11 round mag i am sure it is the 12. On one side it will have odd number that count up to 11 and this might be what is confusing you. If you turn the mag to the other side you will see that it has a spot marked for the 12th round. Yes at first they are hard to load all the way and while i am at the range i only load 10 at a time or 5 if i am going to groupings. The only time i load all 12 is for defensive purposes.
Hope this helps you out
Adam