Questions about hidden content (loaded with spoilers)
 

I've been wondering if there is hidden, small content in the game.

Like winning several poker matches against the same opponent gives a special item or a bit of information (seen a counter for the confrontation so I wondered)

Spent two nights fighting wraiths with the Scoiatel camp near the fog (Iothrel path) in hope something would happen…

I found a key for a hidden door in the catacombs while going in the mines with Saskia, trouble is no time to exploit.

A number of items are registered as quest items while I have no quest for them. Are they to appear for other paths?

The key only has its use when you´re on the Roche path I think.
Similarly, when you´re on the Roche path you may find locked containers or doors which can only be unlocked when playing the Iorveth path, etc.

There´s also an optional encounter in chapter 3 which either ends up with an opportunity to respec Geralt´s skills or probably the toughest fight in the game.
Needed item is straightforward to acquire when on the Roche path since you´re led to it by a side quest, but on the Iorveth path you have to find it purely by exploration.

That may be the case, or you may run into the quest where they can be used later, even in the next chapter. Also, some items may not be necessary to finish quests where they´d apply - I´m pretty sure that if you have Dun Banner cap and cloak they can be used when persuading the wraith in dwarven catacombs, but the quest can be finished peacefully without them anyway.