I've done all of these things quite a lot. With feel parallel or on one foot, without changing the way the force is coming in, it's impossible to resist much force. I can stand that way (parallel or on one foot), and redirect the push, which makes me very stable, and wears uke out.
When I think of redirection of a force I assume a counter force is being applied at an angle to the incoming force as in slipping a punch and applying force to the attackers arm at a right angle to the direction of the punch.
So when you redirect the push are you changing the angle at which the force is being applied to your arm? And can you do that while keeping your arm straight? I would do it by slightly bending my wrist and elbow. With the added benefit of coordinating mind and body (intent) I can reduce the amount of force reaching my shoulder to practically zero. I think of it as dissipating the force as opposed to redirecting it but I suppose both are at work to one degree or another.
From what I have garnered reading Aikiweb posts, the "internal" folks claim that a sufficiently trained person is capable of resisting a full on front push to the chest in natural stance without displaying any outward movement whatsoever. I've never seen it done or fail to be done so I can't say yea or nay at this point. Assuming for the moment that the claims are true, how could you use alignment and structure to explain the feat?