Bran's plot card maipulation just screams trickiness. I'm not sure, though, how changing a plot card during the plot phase in the second 'player action' window would resolve (can it be activated in the first 'player action' window? If so, how would that work?). Initiative has already been determined, so it would not change, I'd guess. The gold and claim values would adjust as they effect the marshalling and challenges phase. But what of the effect? Does using Bran's ability allow for another plot card "when revealed" resolve step in the plot phase?

Also, how are people within the community using Bran's plot ability?

Note that in general, plots do not say "when revealed at the time all players simultaneously reveal plots when the game tells them to during the plot phase;" they simply say "when revealed." So revealing a plot with Bran during a player action window is no different from revealing a plot during the standard framework action window as far as the plot's resolution and characteristics. More to the point:

1. The new plot becomes your "currently revealed" plot and the last one goes to the "used plot" pile.
2. Since the new plot is your "currently revealed" plot, it is the one referenced whenever the game looks at your plot card - like for claim value during Challenges or counting gold during Marshaling.
3. The plot text is active, just like the plot text on any other revealed plot card.
4. The plot was indeed "revealed," so the (active) "when revealed" text does activate as a passive effect initiated by the card effect (rather than the standard game effect) that revealed the plot.

Now, as you noted, the process of counting initiative and determining the first player are separate game effects that share the framework window in which plots are revealed by the standard game effect. When a card effect like Bran lets you reveal a new plot card, those separate game effects are not resolved anew because there is no card or game effect initiating them. But other than that, revealing a plot card with Bran is no different than revealing a plot card as part of the basic rules of the game.

As you note, there are two Player Action windows in the plot phase: one before the framework window in which plots are revealed (and initiative counted, first player determined, etc.), and one after. Bran, as a "Plot" phase action, can be used in either one. In fact, more people I know use him in the first one just in card their opponent would reveal a plot that would kill him as their plot for the round. (Note that when you do use Bran in this "pre-plot" window, the plot you reveal with him is not your current plot for very long....)

Plot manipulation is a nice trick, but not a particularly robust one in the LCG right now. Some of the most common uses for Bran that I have seen are:

1. Reveal "Valar" in the pre-plot window, then reveal a higher gold/claim plot for the standard plot so as not to be stuck with the 2-0-0 plot stats for the rest of the round.
2. Reveal an important "when revealed" plot in the pre-plot window, then reveal a steady constant-effect plot for the rest of the round. (For example, reveal "A Time for Ravens" pre-plot to search your deck for a Black Raven, then reveal A Song of Summer for the regular plot to get the +1 STR on all your characters the same round that you play the raven.)
3. Cycle through their plots faster, especially when they use their plot effects for board control (for example, the City plots from the King's Landing cycle).

The first two are nice, and certainly fun in a tricksie sort of way, but they aren't really game changing 90% of the time. That third thing is probably the most powerful potential use for an ability like Bran's. The difficulty being - which plots are you putting into a deck to control the board that aggressively?

Wow. Detailed and helpful as always, Ktom. Thank you! How is it that you know so much about this game?