The thing is though Kirk was given two important missions that could have started interstellar war with the Romulans and Klingons.
Umm, what were those?
In "Balance of Terror", Kirk had no known mission - he just helped out when the RNZ outposts came under attack. He did so in a region of space that had been neglected for a century, and nothing suggested the
Enterprise wasn't a continuation of said neglect. Nor was any mission involved in "The Deadly Years". So that seems to leave "The
Enterprise Incident", and there's no particular reason why a powerful and valued ship should have been sent on this mission of subterfuge. I'd rather say Starfleet checked the box for "expendable" there...
In "Errand of Mercy", war with the Klingons was reality already, and Kirk's ship was being sidelined, sent to deal with a primitive planet with orders to avoid combat. All the other episodes had Kirk stumbling onto Klingons without any Starfleet preplanning.
..why[..] didn't Spock or Scott suggest calling on reinforcements, a bigger and better ship to handle the gravity of the situation?
We have no support for the idea that calling of reinforcements of any sort, big or small, would be possible in the TOS universe. To the contrary, even calling home base for
instructions may take longer than the adventure allows for! So such things tell us absolutely nothing about the relative worth of Kirk's ship. They merely establish that the ship operates outside safety nets - but not that this would be because she's expected to cope, as opposed to her simply being expendable.
When the M5 was destroying Constellation ships left right and centre why didn't they call up a superior ship to challenge it?
What, and ask M5 to wait for two weeks for said ship to arrive? The entirety of TOS stands proof that summoning even the humblest of transport ships is a matter of careful organizing and lots of waiting.
Also why did the female Romulan Commander think the Enterprise was such a prize? Why would its capture bring her such glory?
Because she was the only ship to have fought and defeated Romulans in the past hundred years?
And remember that
that bit happened in an episode based on a movie about a lowly destroyer escort, the least impressive of all warships, defeating a submarine. A humiliation in battle need not be the fault of a top-of-the-line enemy supership in real life, so we have no real reason to think this would have been the case in Trek, either.
Timo Saloniemi