We felt that this change to M'uru's spell interruption would ease some of the frustrations certain classes were having with the encounter.

Some accomplishments (i.e ZA bear mount) will have a limited time in where you can get them, yes, but generally speaking there are quite few of them in the game. For players who are focused on romping around on a bear, there will be another one in Wrath of the Lich King.

As Tigole put it, does a polar bear sound cool?

We currently have no plans to reduce the reset timer for Karazhan. If you need more gear and/or Badges of Justice, there are a number of heroic dungeons to roll through.

There are currently no plans for any changes in this regard, but we will of course let you know if it is ever decided to make any such changes.

A games master has already told me that this is a legal action and that he could do nothing about it.
This is correct. It is currently considered a legitimate means of playing multiple accounts.