There are dozens of ways of fitting a sling to this system. I'm not convinced that the rear swivel at the end of the receiver rather than the stock it THE solution, but it's popular and I use it on one rifle.
Removing the lower receiver extension is an easy task. The replacement end plate just substitutes for the carbine end plate and works fine with a standard, fixed stock as well. You should download the Army TM9-1005-319-23&P for details on removing the stock. The rear takedown detent spring is under there and the buffer retaining pin is under the lower receiver extension. The buffer retaining pin will leap violently from the lower receiver once the extension is backed out a couple of turns; the detent spring will squirm out when you remove the end plate. Both are easy to control, just be aware. (They're also High Use replacement parts!)
There are probably a dozen different end plates with single, double, full loops, etc. for this purpose. I have the single hole versions for "HK type" clip on slings mounted here. But I only mount a sling there on one rifle at present. The HK hook type allows the carbine on the left to be single point slung and a few tries will convince most folks that it allows for little control unless holding the weapon.
Note the Magpul stock on the right uses the slot for the sling.
Sliding stock sling can also be mounted like this using one buckle as a "keeper" on the opposite side. This is a Blue Force sling on my M&P 15-22 but it works the same on any carbine stock like the one on your M&P 15T. Both sides shown.
Carbine front swivel can use a standard side swivel attached to the barrel,or with one that clamps to the rail on your 15T.
A Blue Force sling and some sort of rail mounted front swivel is all ya need to start. You can figure out later if you want a swivel between the lower receiver and stock and if so what type later on.
-- Chuck
