I'm hoping somebody can help me with the final piece to my newly installed flooring. I installed 1/2" x 5" engineered. I have about a 5.5" space between the last plank and the edge of the floor where the stairs start. The stair nosing is only 3" wide all together, and probably only 2.5" of usable area before the bullnose. How can I install this securely using the tongue and groove of both?

Won't be able to.....working towards the staircase is always a crapshoot on whether or not boards will line up with moldings. You'll need to go ahead and dry-fit the nose, chalk a line, and cut your last plank to fit to the line.....sounds like from your description you're going to have about a 3 1/2" wide plank. Then flush the stairnose up to the cut edge, glue it down and face nail it to be sure it's secure. If you really want to have the nose connected to the flooring, you'd have to do some routing on the last board and join the two with a slip-tongue or biscuit.

How would I face nail? Really not sure what that is, as this is being glued directly over concrete slab. That is really my concern..... how secure it is. Being that the nose gets so much traffic, I didn't think just the glue without having at least the tongue in the groove for extra support would work.

It will be fine. just glue it if it comes off then glue it back again thats about your only option and is also how they are usually installed. Its too late to change anything now.

I sure wouldnt want the liability installing it that way.
I shoot a facia board into the riser . This squares up the step, but also gives me something to anchor into.

Am I seeing the pictures right?
It looks like there is wood on that riser ( behind the backer board ), to topnail the nosing into.

If that nosing ever came loose just as my rotund ex wife stepped on it, I would be eternally grateful
Best thing to do here is anchor it with countersunk concrete screws and fill the holes with some matching putty or small wood plugs finished to match. Not going to be invisible but at least it would be safe.

BR111 makes a 5 1/2" wide nosing but it's designed for solid 3/4"

http://www.br111.com/molding1.html

I'm thinking you could retrofit it to work with the current floor. The question is if the distance from the last plank to the tile riser is the same dimension on both ends.

You are correct sir....... there is tile, then backerboard, and then wood. Not actually part of the riser per say, but I could definitly nail into this. What is the best type of nail for this?

Jesus man the thing if glued right is not going anywhere, I dont care how fat your wife is, she wouldhave to be a 2500lb rhino with steal cleats that took a running start and slammed on the breaks to even come close If glued correctly with an quality adhesive once the glue sets your going to need a hammer and a chisel and about 30 minutes worth of hard work to get it back up in tiny peices.
btw shooting a finish nail in the riser will help hold it till the glue sets but its the adhesive thats doing the real work.

Each to his own, I guess, I wouldn't put my faith on glue alone for a nosing. When you step on the edge of the nosing when going upward, the vertical force applied is far greater than I would trust glue to hold, especially after years of daily traffic.

Absolutely Dennis, There's no way I would trust a nosing with glue only.

I use finish nails , 3 times in length of the wood thickness, ( 1/2" wood =1 1/2 ").
Make sure to drill pilot holes first, so the nosing doesn't split.

I personally like the idea of concrete screws and then using a little putty. I have a plug cutter, may even try that on a scrap piece and plug it after the counter sunk screw.

The plug makes sense to me.
But get out the trowell and putty over a screw head ?