Brad, when I said "free tenon" I mean the same as the "spline" that you have pictured. I agree with the bare metal not being the best solution. A couple lag bolts or timber screws in addition to the free tenon/spline wouldn't hurt.

As far as putting the screws through the post and into the beam at an angle, while better, it is still not as good as toe screwing.

The timberlinx seem like a good idea. I've seen some home-made timber fasteners that look like a super-sized bed bolts. If timberlinx cost $25 you are probably better off purchasing them over building your own, but really they do essentially the same thing as a free tenon/spline, so that's what I'd do.

For the housing (shelf) I'd go at least 3/4" but no need to go more than 1"