J, welcome. I'll suggest to you that almost any room can accomodate two small surround speakers such as the excellent QS4s and that this would be a more significant advance in your listening enjoyment than a center speaker. If necessary, your M3s can form a "phantom" center at least temporarily, which you may find to be satisfactory.
As to the suggested use of an M3 inside the cabinet for a center, none of the alternatives that you propose sound very good(possibly literally). The first point is that the grill cloth on the cabinet isn't likely to be very transparent acoustically. Then, it isn't the port which would really be the problem inside the cabinet, assuming that it had at least a couple inches of "breathing room". The M3 midbass cone would be more of a problem with resonances and diffractions caused by the cabinet surfaces. Maybe if you'd describe the space available in a little more detail an alternative placement could be suggested.
