Great questions. With the GnP in UFC 4 we are trying to capture the severity of the situation. We want the players to have that "It's on now!" moment when going to GnP which is why we have them shift into very offensive and defensive roles when the moment starts. We have struggled a lot with having animations which are both fluid and reactive in the grapple game as we have two players whose moment to moment actions don't always align and we need to constantly bring their respective animations into alignment. What you are playing is the best balance we have been able to strike to this point but we are constantly trying to refine it.
With regards to your questions about the transitions. When the moment starts the Dominant fighter is able to do some transitions immediately while the submissive fighter must wait for a small amount of time before they can transition, the dominant fighter can extend that time by being active in the position and landing strikes. This timing is indicated by the same HUD element which is used to indicate advantage in the grapple game, when it moves into the subs color the sub is able to transition.
For the Dom to initiate a transition he has to hold RB, this was to prevent accidental inputs from forcing players out of GnP when they didn't want. If the sub is holding RT when his transitions become available he will automatically do the base pull down transition, this was to allow casuals a way out even if they just turtle during the GnP moment. Holding RB opens up their full suite of transitions.
Also note that not all fighters have all the transitions and some are contextual. Submissions for example require that you get a stun event on your opponent in order to apply them, this was because we didn't want people using GnP transitions as a cheezy way to get to subs, if you go to GnP you better hit your opponent. Other transitions require a stamina or health advantage.
We did hear a lot of complaints about the accessibility of the GnP transitions in the Beta which is why I added the HUD in the earliest patch I hope it helps.