Re: final lock doesn't work
I'm with Chris on this one - my opinion.
It's true, there are multiple centers, but when you see these centers affecting other centers (such that a majority of the opponents body comes under your control - however you want to phrase that) in Aikido waza one has to take into account that the various physical properties (inertia, centrifugal force, gravity, etc.) of motion are playing a huge part in this type of control. Without the physical properties of motion playing this huge role, as when someone has already stopped moving because they are laying prone on the ground, the amount of centers that you can affect from any single center goes down significantly.
This is why one feels more controlled as Nikyo is being applied than when one has taken ukemi and is sitting or laying at the bottom of the Nikyo motion - where motion has stopped. This is all compounded when one has stopped and laying on the ground prone. Why? Because it becomes next to impossible to mechanically control the two major axes of motion (i.e. the spine and the hips) from a single center (e.g. the elbow as in Ikkyo) when the action has stabilized somewhat. The forces necessary to align the centers, such that one lock can lock several other centers, are absent; so then is the necessary restriction on alternate alignments, so then is the capacity to use one to control another. This is why BJJ is so great on the ground - you learn how to move one or both of these axes; you learn how to control them in someone else; and you learn how to use them against someone else.
Aikido "pins," in my opinion, are better seen as setups for breaks, and/or holds from which you can strike (armed or unarmed), and/or exercises meant to develop a sense of grounding/kokyu, etc. One should not expect to pin anyone - in complete control - with standard Aikido Kihon Waza pins (in my opinion).