Hey Zizou, firstly cheers for the friend request last night! I'm playing the Absolution story mode on Purist difficulty and can confirm that you do still get Instinct mode (I wouldn't have got far without it!), but with no indication on the HUD of how much you're using and it really doesn't last very long at all that difficulty level, usually just enough to slip by the odd guard occasionally.
I've still managed to get Silent Assassin ratings on some of the levels so far, so Purist is not insanely difficult, though you have to be really cautious and observe the environments well enough to get an idea of guard movements/locations. Even then you'll get the odd WTF surprises that have seen me darting for the nearest cover to hide behind!
I got sent another Contract competition invite from darkmorgado yesterday, to a map created by someone called 'madtheswine' (fellow EG'er??). It's a fun little sniper challenge in Chinatown which I've forwarded an invite to both you and fletch7100, who I see has been playing this too