The ending made me groan in disbelief.

Given Zellner's strong resemblence to Poirot, I can well believe it!
After all the time Legrand spent chasing the Raven, his reaction was a bit unbelievable, too.
So many things were glossed over, such as how the bomb got in the basement.
The mastermind had the blueprints, sure, but when did he plant it and where was it?
Did I just miss that?
How did Alex get away?
(I know, we were supposed to fill that in for ourselves.)
Considering how unlikely it is that the sort of shirt whatsit was wearing would hide a vest which could stop a bullet, I'm surprised the killer didn't shoot whatsit in the head and Game Over.
Would a butler really be considered a representative for his dead boss?
Doesn't seem very likely to me.
I guess this is where suspension of disbelief comes in.
I think you're right about the game going downhill from a strong beginning.
I don't know...maybe they had a professional writer script the first episode and perhaps sketch out the rest, then decided to take it from there without him.
They gave a lot of attention to Captain Di Conti's backstory, for instance, then just dropped him.
Some of the twists and turns were pretty decent but I had a fairly good idea whodunnit long before the mask was removed.
I suppose they took the spotlight off "whatsisface"

since they wouldn't be able to have
an exposition by the detective at the end of the game that would wrap things up and explain exactly what happened, all the suspects gathered around with the finger going from one to another until it landed on the killer,
but it did leave me with a disjointed feeling.
As you say, as if the game was fractured.
Gil.