The trap damage is limited, so you can just trigger it and cope with the damage. Depending on your health you might need to use a potion or two to avoid dying but either way it will eventually burn out.

Eventually, the goblins will turn hostile towards you and you may just have to kill all of them anyway if you take on one of the side quests that involves the troll king. I didn't bother to disarm the trap myself as the damage wasn't a very serious one. Wait till you have full health before opening the chest and you can survive the trap.

I believe there must be a free rune for it. Click & drag to your inventory.

Because, I have founded so many runes for free,
I can almost spell, every spell (accept a few)
Maybe 2 or so, are buyed.

Have you checked the chests in the: Mages-Room, in the upper-part
of the Castle ? Lotsa stuff to take for free. And there's more.
Check all caves around the Ice dragon for example.

Use LEVITATE, go across, then push yourself up against a wall.
You're now flying higher. maybe you can now access the other caves ???

LOL

Having read this topic and made a suggestion (just edited out) which I just verified would not work, I also tried something else when I reached that point:

I deliberately did not use the scroll (which I had) but lockpicks to disarm the chest; my skill was 44 at that time, and it worked.