Point 1: the example was an example[2]
You wanted an example where the Crown has an option other than taking the advice of the elected leader. I gave you one, and quite a recent one. The fact that it is no longer used does not obviate its existence. A gun in a holster is still a gun. A sword in a scabbard is still a sword. The Sergeant-at-Arms of the Canadian House of Commons is a purely ceremonial post...until it isn't. Ceremonial powers are ceremonial powers...until they aren't.
Point 2: nomination vs appointment, must vs can, removed vs changed
The legal theory is that the Crown is supreme: all power flows from it, all authority derives from it. Consequently the Crown is not told what to do, it is advised. This is why I have been careful to use words such as "nomination", "can", "changed" instead of "appointed", "must", "removed". Of course as we both know, the everyday reality is entirely different: the Crown is a legal fiction, the monarch is a figurehead, real power lies elsewhere. But the legal theory remains in place, and (as I explained above) legal fictions are not fictions in law - quite the opposite. In law the PM does not appoint the GG: the PM advises the Crown and the Crown appoints the GG. And at times such as the Dismissal, the law counts...as Whitlam found out to his cost.
This whole concept underpins the Westminster System and failure to understand it results in many errors. Consider a recent case in the UK concerning the imposition of press regulation via the Privy Council[1] rather than via Parliament. That such a thing could occur was hotly disputed by press editors, because the press editors believed the Crown was a figurehead and it could not happen. But in the legal world of the law, the Crown was supreme and it could happen...and it did. Now that they have realised their mistake, Parliamentary action was scrambled to circumscribe it by defining it, and they may yet succeed in overturning it. But the fact that it went as far as it did illustrates my point.
And arguably, the fact that the Dismissal took place at all illustrates it even further. Because it happened IRL.
Notes
[1] see
http://en.wikipedia.org/wiki/Royal_Charter_on_self-regulation_of_the_press
[2] Incidentally, why the scare quotes around "precedent"? It's factual and it happened.