[Wulfere]

Or what spell is it that mages cast to cause instant death with a fortification save that a character with 150-300 hit points is unable to meet?

That would be Disintegrate or for a Cleric Destruction. The saving throws against a high level mage or cleric is adjusted up because of their skill. The saving throw for a High level cleric could be very high indeed. Partly because the saving throw you must make is dependant on their Intelligence or Wisdom bonus'.

Allright... So, im immune to mind effecting spells with this golden circlet...but fear still is quite capable of striking me?

get a "lesser Belt of Guiding Light" it grants immunity to Fear

Or why is it that i can cast greater stoneskin and get no messages on incoming hits telling me xxx points of shielding left? Or why is my attempts to resist spell roll not displayed?

You should check your game settings on the options screen. "Game Options" then "Feedback options" make sure the targeting feedback is "always on". That should help with some of your troubles.

Or where is my cloak of non-detection?

If there isn't one in the game you could always use the toolset to make yourself something like a cloak of non detection.