You say you don't have time to play these games... but you have time to do these kinds of lists?!
I'm an extreme INTP too though so I understand!
The game time thing? Sorry, but who keeps track of that? Impossible to do so I will leave that part out.
Also, I think there's a distinction that needs to be made between 'legitimately' difficult and difficult through poor design (which I have designated as 'illigitimate' difficulty). Nearly all third person games have some illegitamate aspects like pixel hunting, wandering around rechecking areas after every event trigger etc so it's rare I will class them as 'entirely legitimate). If those aspects are limited I will designate them 'mostly legitimate'.
The games I have played from your lists I would rate thusly:
The Next BIG Thing: 5 (mainly legitimate difficulty)
Black Mirror 2: 6 (difficulty is mainly illegitimate)
Machinarium: 8 (totally legitimate difficulty)
Blackstone Chronicles: 6 (mostly legitimate difficulty)
NIBIRU: 6 (difficulty mostly stems from boredom at tedious tasks which has you reaching for a walkthrough quickly so as not to waste too much time so has to be illegitimate)
Syberia 1 & 2: 4 (mostly legitimate)
Sanitarium: 5 (mostly legitimate)
Last Express: 10 (legitimate)
Titanic: adventure Out of Time: 8 (legitimate) (this rating is for the perfect ending)
Post Mortem: 7 (some illigitimate bad design but also some very tough ligitimate puzzles. But interspersed with some simple stuff)
Still Life: 5 (mainly legitimate)
Still Life 2: 8 (a lot of illigitimate stuff and terrible frustration with the controls and interface)
Art of Murder: FBI Confidential: 5 (almost entirely illigitimate. Used a walkthrough a lot through sheer boredom)
Axel & Pixel: 3 (entirely legitimate. Without the 'action' levels this would be a 1)
Kaptain Brawe: 3 (mostly legitimate)
Dark Fall: The journal: 4 (entirely legitimate)
Dark Fall: Lights Out: 5 (mostly legitimate)
Dark Fall: Lost Souls: 6 (50 / 50 legitimacy)
Paradise: 7 (quite a lot of illigitimate difficulty from the poor interface)
Next Life: 8 (a lot of walking around the same area over and over looking for what has 'changed', with no clues, so mostly illigitimate, plus challenging arcade sequencies)
Return to Msyterious Island: 3 (mostly legitimate)
The Lost Crown: 3 (mostly legitimate but some repetitive walking around looking for 'what has changed')
Scratches: 7 (Mostly illigitimate)
The Experiment: 9 (entirely legitimate)
This is only games I have played through to completion and feel that I remember well enough to judge them fairly. No doubt I have missed games as I skimmed down the list so I may post again if I notice any and if I can be bothered.
