When President Obama attended the G8 meeting in Ireland in June of 2013, the US Navy parked two super carriers (and their accompanying fleets) off the Irish coast. And we're friend with them.
But that's the exact opposite of the Khitomer situation! Imagine a conference held in Ireland in which President Reagan and Premier Rasputin discuss how to deal with the recent assassination of Gorbachev by US special forces in the aftermath of the Chernobyl explosion. Sure, both sides would wish to bring in security. But neither the parking of supercarriers nor the admitting of SEAL and Spetsnaz forces next to the conference venue would make any sense: the supercarriers would just be sitting ducks in the very likely scenario of WWIII starting (there being no sense in launching the air wings against Ireland!), and the special forces would be counterproductive to the extreme.
As far as the conspirators were concern, there was no real reason not to have a fleet presence around the planet
Sure there was. They were the ones who knew that the assassination assuredly would take place, and the war would start in a few hours. Everybody else might have believed otherwise and agreed to a prominent starship presence, but the conspirators didn't want to die - they'd allow for secret getaway vehicles and nothing else (while secretly from each other amassing fighting forces that would make short work of the other side
and his getaway vehicle).
Although it should be pointed out that we have no idea whether Chang and the Klingons were involved in the assassination in any way. Killing of Gorkon had been a joint operation, but killing of Ra-Ghotarei might have been a Starfleet-only (or, more accurately, Cartwright-plus-Nanclus) scheme, intended to cast all the blame on Klingons exclusively. Chang would be chasing Kirk for completely separate reasons already, not wanting him to sing about what had happened to Gorkon.
The only thing I can possibly see preventing the Romulans from commiting the strike I mentioned is the possibility of a firm Federation/Klingon Alliance against them for the attempt. That would give any opponent pause.
Indeed, there must be a reason why there has only been cold war so far, and why it's now going to go hot. Something big must have changed. And this something could very well be the Romulans declaring they are now the allies of the Feds and enemies of the Klingons (to the Feds) / allies of the Klingons and enemies of the Feds (to the Klingons).
No need to get involved beyond that. The two sides would independently decide now is a good time to go to war, what with the Romulans abstaining. And Nanclus could just sit back and coordinate.
Again, an outside attack at Khitomer would be a welcome event, as it would probably prevent the UFP/KE war and possibly even start an alliance, no matter how short-lived, on avenging the deed. The only ones who wouldn't welcome this outside attack would be the conspirators who wanted each other to look bad, not some third party.
Timo Saloniemi