It can often depend what country your bag was made in. Actually , I have seen many genuine quilted Selmas with six stitches under the handles. Your bag actually appears to be authentic to me and has all the correct markings. Here is a quilted Selma you can compare yours to that is listed on ebay by a Top Rated seller that has thousands of positive feedbacks for selling authentic bags for many years. Notice that this Quilted Selma, also has six stitches under the handles. Not every bag is identical. There are variations from year to year and different countries manufactured in.
The quilted leather feels thinner than regular leather. It needs to be thinner so they can quilt it. So it is going to feel different than what you are used to. Also, Michael Kors has been packaging their bags in plastic for some time now. Not unusual at all. At one time they all came wrapped in tissue but this has changed the last couple of years. Their gold hardware is a coated / gold plated metal. It should not feel plastic. That raises a question. Are you sure it is plastic? because it should definitely be metal.
Here is a link (below) from a reputable seller that you can compare your bag to. While I believe your bag is authentic, it would help if you could tell us where you bought it and provide a link to the listing, because there are super fakes out there. Your bag looks authentic to me. The only thing I question is what you are stating about the hardware being plastic? I suggest taking it to a shoe maker and ask them what material is the hardware to verify what it is, exactly. if he determines it is indeed plastic, then it can not be real. Just make sure you are correct about that, because everything else looks correct on this bag. IMO.
http://www.ebay.com/itm/261978614144?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649
Here is a quilted Selma from NORDSTROMS that absolutely does not sell fakes. It too has more than four stitches under the handles. Here it looks to have five. Like I said, much depends on the year and country manufactured in.
http://shop.nordstrom.com/s/michael-michael-kors-large-selma-quilted-leather-satchel/3829284