Port placement directly behind the sub

[Bwcad1]

boxes aren't large enough to really worry abour cancellation, and the port being opposite the sub is fine. It's not really about the wave direction so much as restriction of airflow to affect damping of cone movement.
What you do want, is half the diameter of the driver's cone between either end of the port and any obstructions.

In short on cancellation, the old argument is to angle the side of the box opposite that of the driver(s) to avoid waveform cancellation. The reality of the situation is that the box isn't big enough to even affect a harmonic of the frequencies generated by a subwoofer, so it's essentially a moot point, and just done to make a box fit in a given location now, or to appease people who don't really understand acoustics.

Exactly. Even the loading issue only comes into account when running high power around the limits of the mechanical suspension. The suspension of a driver should dampen any side loading until you start reaching the mechanical limits of that suspension.

[GlassWolf]

honestly, when I ran the Festiva's two CV 12" subs on the Orion 280GX ported, I never had an infrasonic filter of them, and they never unloaded on me, and I hit some hellishly low notes on that setup. Low enough that I couldn't hear any sound from the stereo, but the steering wheel would be vibrating about 1-2" like a blur. (Floyd's DSotM heartbeat, and KLF's White Room CD were the two that were the worst.) So yeah, I was driving 160 watts RMS per sub, on subs rated for 250 watts continuous (125W per coil)

It's a lot to do with the entire setup really. Hard to use blanket statements for some things.