Fair enough. And I certainly have nothing against the idea of the Ambassador's retinue being 70 strong...
Whether that would mean 70 diplomats, or two diplomats and 68 Marines to guard them, remains to be decided.
The Federation came to dictate terms, that much seems obvious from the teaser. This is also reflected in the later actions of the heroes, as they do not hesitate to point very big guns at Anan's world with the express purpose of slaughtering random people until Anan is negated as a threat. A large contingent of diplomats would appear superfluous for the purposes of dictating a "cease and desist" treaty, or even for securing the "treaty port" discussed...
Timo Saloniemi
I thought this over, and it occurred to me that the Federation probably sent a substantial support entourage with Fox in the event of any contingency.
I assume that the Federation dispatched a starship with the stature of the Enterprise because they evaluated the situation and decided that if it warranted a high-profile mission, and they picked an diplomat and support staff to be prepared for anything.
Consider this passage from the end of the teaser and the beginning of act one:
UHURA: Captain, message coming in from Eminiar Seven. Sir, it's code seven-ten.
KIRK: Are you sure?
UHURA: Positive. It repeats over and over.
FOX: Is that supposed to mean something?
KIRK: Code seven-ten means under no circumstances are we to approach that planet. No circumstances whatsoever.
FOX: You will disregard that signal, Captain.
KIRK: Mister Fox, it is their planet.
FOX: Captain, in the past twenty years, thousands of lives have been lost in this quadrant. Lives that could have been saved if the Federation had a treaty port here. We mean to have that port and I'm here to get it.
KIRK: By disregarding code seven-ten, you might well involve us in an interplanetary war!
FOX: I'm quite prepared to take that risk.
KIRK: You are! I'm thinking about this ship, my crew!
FOX: I have my orders, Captain, and now you have yours. You will proceed on course. Achieve orbit status and just leave the rest to me. You're well aware that my mission gives me the power of command. I now exercise it! You will proceed on course. That's a direct order!
(Fox exits the Bridge.)
SPOCK: In view of code seven-ten, Captain, may I suggest--.
KIRK: Yes, Mister Spock. (speaking to intercom) This is the Captain. Condition Yellow Alert. Phaser crews stand by. Deflector shields up. We're going in. Peacefully, I hope. But peacefully or not, we're going in.
Captain's log, stardate 3192.5. Now in standard orbit around planet Eminiar VII. My orders are clear. We must establish diplomatic relations at all cost. Preparing to beam down to planet surface.
* * *
The Federation doesn't know much about Eminiar and Vendikar, but given the obvious use of subspace radio technology by these worlds, it is possible the Federation may have intercepted some signals from either or both of them. This interception could have been from the ill-fated Valiant or from subsequent unmanned probes that entered Star Cluster 321. So it's a safe bet that the Federation had mimimal knowledge, but was not devoid of knowledge.
As for the motivation of the Federation and what the Enterprise was sent there to do, Fox said that in the last 20 years "thousands of lives have been lost". So, to date, they know that thousands of people (presumably at least some civilians, possibly most of that number) have either vanished or been found dead in the vicinity of Star Cluster 321. This may or may not be a direct result of encounters with Star Cluster 321's inhabitants. It could be pirates from other nearby systems, it could be the Klingons, or it could be natural phenomena along poorly placed shipping lanes. We don't know.
It seems logical to assume that the Federation figured out that people were either being killed by the inhabitants of Star Cluster 321, or these innocent space travelers were being killed because they were using poorly place shipping lanes that avoided Star Cluster 321. It could be either one. But the Federation had to know that at least some inhabitants of the cluster were very advanced. So if the situation had become enough of an annoyance to the Federation, they're going to do something about it. Namely, they're going to re-route the shipping lanes and make these advanced worlds into useful ports for safety reasons.
I don't see the show's teaser as laying out the Federation as "dictating terms", though. No, I see Kirk and Fox being sent to 321 to troubleshoot. They at least know that Eminiar VII and III were at war when the U.S.S. Valiant came to visit. And Kirk makes sure to ask "you're still at war?", a little bemused but not totally shocked. So the combination of Fox and the Enterprise was probably sent to ensure that whatever problems may have existed between these two advanced worlds would be addressed decisively so that the port(s) could be secured.
At least, that's how it looks when I watch this show.