Gandalf, Moria, and Arrest

[Belegorn]

A few things.

Regarding the staffs of the Istari, if they were intricately linked to a wizard's power, why did Saruman not take Gandalf's staff when he imprisoned him? I do not recall ever seeing a Maia or Vala having the ability to take or diminish the powers of another Maia. Has this been accounted possible in the annals of Arda?

I'm not certain of the idea of the native powers of the Istari being restricted by their embodiment. Rather their embodiment was meant "to treat on equality and win the trust of Elves and Men" [UT; The Istari]. I think they were not to use their powers to have the Children of Eru heed them under duress and to not appear too grand, thus coming in humble bodies, that is, as older men.

Originally Posted by Appendix B: The Third Age

they were forbidden to match his power with power, or to seek to dominate Elves or Men by force and fear.

Originally Posted by UT; The Istari

their emissaries were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men and Elves by open display of power, but coming in shapes weak and humble were bidden to advice and persuade Men and Elves to good, and to seek to unite in love and understanding all those whom Sauron, should he come again, would endeavor to dominate and corrupt.

@ Moon yes they feared Moria and Boromir did not wish to go to Lothlórien either.

Your 2nd quote by Aragorn again does not sound like his concern was specifically a death sentence.

"I say to you: if you pass the doors of Moria, beware!" [FotR; A Journey in the Dark]

"Did I not say to you: if you pass the doors of Moria, beware? Alas that I spoke true!" [FotR; Lothlórien]

What was it he was speaking of when he said beware? Was it "imminent death" or something else which just happened to lead to it?