Reiko's fall, a little bit too sudden?

We all remember the story of the previous antagonists. Before we take on them we had to clear a large number his army first. Missions like Raisu Palace/Destruction's Depths/Gate of Madness are for this purpose, but as for Reiko, before we take on her alone, all we did is clear some Tengu's ambush. Why would she choose to face Miku and us alone?

She wasn't expecting us to jump them and she doesn't exactly take us on alone. Didn't you see the Ministry minions that jumped in during the fight? Still, it's stupid that she went around so lightly guarded. Arrogence of power, perhaps?

Indeed. Most of the other villains we've taken down at a moment where they've just amassed a large army for one reason or another, or we've hacked or slashed our way into their inner sanctum to get to them. In this case, it seems Reiko was relying on secrecy for security, and that means keeping a low profile... and the more guards you have, the higher the profile.

There were some there, but, well, not enough. :P

The bigger break in the suspension of disbelief on my account was that after

Drax, your spoiler tag isn't working. :P

No, we didn't face Reiko too soon. Yes, for the other cases we wade through the enemies' armies but that's because:

1) Those armies were deluded innocents like the MoP. We had no reason to spare the titans, shiro'ken, margonites/demons, and destroyers.
2) Those armies were placed between us and our enemy.

Reiko is a guild wars 2 mesmer! Cloning herself.

I think make the previous quest about MoP army rather than Tengu would make more sense.

Nah. The MoP is too much of an insidious evil to make a direct attack on something like the Monastery. Most of its members are still fundamentally decent people - that's not to say Reiko had any shortage of fanatics who can be sent to pick fights and then claim that they were acting in self defence, but that wouldn't fly with something as audacious as a full-scale assault on Shing Jea Monastery. Being conveniently elsewhere after inciting someone else to attack, though, is something they could pull off.

Not exactly the same plot, just let us fight a bunch of MoP, they can be the most zealous ones.

That fight is critical to the MoP's ideals and why they are bad. They took their time getting to the fight so they could enforce their ideals about 'cleansing' the Tengu. Another quest fighting the MoP mobs after the Tengu would have been better for the story than leaving the Tengu fight out.

The main point of the missions that have us delve through enemy forces, other than them being threats pure and simple (rather than people who are blinded by ideals) is to remove the actual threat (or at least a damn good chunk of it).

The MoP "needs" to survive to help further the isolation-xenophobia that Cantha becomes under Usoku.

Similarly, I wouldn't doubt that Ashu becomes a mentor to Usoku.

I think that having a quest where you're wiping out dozens to hundreds of MoPs would go against the entire point of the WoC storyline - they spent time making the PC being someone who does not go kill innocents just because they're told it's "right" (which never really happened with them being wrong... And to those who dislike the personality given to the PC and arguing that it's different from the PC at other times - that's because this is the first real time they gave the PC a personality beyond being either a grunt (Prophecies), middle man (Factions), or temporary leader (Nightfall and Eye of the North)).

Going out to kill dozens of potentially good people who may merely be misguided before killing the one misguiding them would go against the entire purpose of the character development.

Part 3 was wrapping things up too quickly, but taking a fight to the MoP before Reiko would have made it worse.

Except the key word for even the most fanatical of the MoP is plausibility. They can get away with doing horrible things to Tengu "in self defence" because what few human witnesses there are can be written off as Tengu-sympathisers. They can also get away with the same on a smallish human village (clearly, anyone who survived the massacre did so because they were spared by the tengu to spread anti-MoP lies!).

A full-frontal attack on Shing Jea Monastery, however, is just too public. For an organisation that relies on the good will and support of the common people (which, largely, our previous adversaries have not) attacking the monastery directly would have had too high a risk of a headmaster or some other respected personage escaping and reporting that it was the MoP that was responsible. Far better to incite the Sensali and then let themselves be "tricked" into withdrawing as they sit back and let the Sensali actually commit the massacre they're going to be blamed for.

Yeah, so I said they can make a new plot for the quest such as eliminate those protesters.