Is it possible that Worf was denied his ambassadorship?

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Worf could have just done everything he did in NEM, but in civilian clothes instead of a uniform!

Except be present on the bridge, during battle, standing behind a weapons console.

I hear he may have had some pull with the captain...



TNG 'All Good Things...' - in the alternate future, Picard, Geordi, Data & Worf were all civilians, and still got to main stations on the bridges of the Pasteur and the Enterprise!

DS9 'The Adversary' - the Founder disguised as Ambassador Krajensky was allowed on the bridge of the Defiant.

VOY 'Caretaker' - Tom Paris was allowed on the bridge & took a station as a civilian observer.

 

[Sci]

Two, as a new member the House of Martok the Chancellor's enemies could easily level nepotism charges against him.

Yeah, but it'd be the kind of nepotism that would be good for the Federation. Remember, if he's the UFP Ambassador to the Klingon Empire, he works for the Federation, not the High Council. So if he has an additional "in" with Martok, that gives him -- and, by extension, the Federation -- more influence over the Klingon Chancellor.

Worf was to be the Federation's ambassador to the Empire so he would have to prove his request against other candidates.

Not necessarily. Why would Admiral Ross have conveyed the idea to Worf at all if the Federation President had not already singled out Worf for the position?

It is possible that Worf served as ambassador for a brief period, but why would that be? K'helar was clearly murdered before regular retirement but is there any reason to Worf would quit outside Martok losing the Chancellorship?

I'd suggest reading the TNG novels A Time to Kill and A Time to Heal by David Mack and A Time for War, A Time for Peace by Keith R.A. DeCandido.

ETA:

Why can't he be a Starfleet officer and an ambassador at the same time?

Isn't that a bit like saying someone could be an active-duty United States Navy Commander while serving as U.S. Ambassador to the Court of St. James's? They're both full-time gigs.

We've seen that Starfleet handles a great deal of the Federation's diplomacy.

On an ad-hoc, when-there's-no-civilian-member-of-the-Diplomatic-Corps-around basis. Not on a conducting-permanent-and-regular-diplomatic-relations-as-head-of-a-foreign-diplomatic-mission basis. That would be disturbingly militaristic of the Federation.

Denied by whom in the Klingon Empire?

To use an analogy, if the US President sends an ambassador to France, does an opposition party in France have grounds to refuse it?

Almost never. And it's very rare for a head of state to deny the credentials of a foreign ambassador, especially from a closely-allied state.