First of all, I think Inquisitor is a great game for its original story idea, even if I cannot quite imagine who can tackle this game at the highest difficulty level at the first playthrough! :-) If anyone has managed, please feel free to post how!!

What I find however a real point of frustration is the very exacting sequence of quests, where one can quite unwittingly and fairly naturally do a given quest in a "wrong order" and find oneself unable to progress the main quest as the result.

I do not mind the idea of breaking the main quest as such - but more so of not warning abou it and closing the remaining game wolrd for it. In Morrowind you could certainly break the main quest, and still enjoy the game - but you were warned immedately, so you could reload and get on with the main quest if you liked.

SPOILERS:

ALPHONSO (from beginning of Chapter II): do not talk to him first, as he is incriminted. Arrest him immediately by talking first to the authorities to get arrest warrant. Otherwise you do not get the Devil's Cavern location, vital to progressing to next chapter. I had to reload for double digit hours of gameplay for that - very annoying.

TINTHAN (end of Chapter II): do not resolve his issue of accusation, without having progressed the main quest to the point of being able to ask him for a counter-favor of stealing the letters. Otherwise this grateful soul will disappear, and progression to next chapter will be comprompised.

This is inspired by "wolfsbane" ingredient suddenly disappearing from inventory (with a reasonable reload time at least) at beginning of Chapter III : shall I need it to finish the game??

Any other fixed sequence quests (even side quests) you may have noticed?

Both issues are not breaking the main quest.

I think I never arrested Alphonso, but still got the location of the Devil's Cavern (not sure if he gave it to me or someone else). And Tinthan will vanish from his original location once you resolved his problem, but reappear somewhere in the city, so you can talk to him again.

SPOILERS!
Nothing happen for me at least not game breaking quest, completing everything with tinthan(ask him to steal and so on) before doing the rank mission with your paladin can break that quest (but it's not a game breaking quest)this is confirmed in the forum of inquisitor and they say if they ever release a patch will fix it.

Regarding Tinthan, he is to be found in Alvaron afterwards, but once has to trigger Chpater III for that as far as I know. Maybe it is possible to prosecute Truncquillius without the letters, but I did not really wish to try.

As to Devil's Caverns... i really got stuck with that one after having played quite some bit of Chapter II - so if there is another way, would be great if someonce could post a hint!