Robert Comsol wrote:
Later, Scotty was able to fix their impulse power: "Engineering to Bridge. Try your helm. You'll have enough (impulse) power to keep her stabilised."
But at this point their orbit had deteriorated to a point where it was only possible to "stabilise" but no longer to "blast out of the orbit".
I think assuming that Scotty meant "impulse" power doesn't match up with the episode.
Let's go backwards and look at what Scotty fixed...
Scotty sets a jumper or two in the Jeffries tube and then tells the bridge they have helm
SCOTT: Engineering to Bridge. Try your helm. You'll have enough power to keep her stabilised.
Earlier, Kirk tells Scotty that they've lost helm and
power as separate items
KIRK: He's cut off both helm and power.
And in the key scene, helm is separate from warp and impulse power
. Helm is specifically identified for "stabilising"
which they do later on when Scotty gives it power. The warp and impulse engines are specifically for getting them out of orbit.
KIRK: Helmsman, stabilise position.
BRENT: Helm is not answering to control.
KIRK: Warp us out of here.
BRENT: No response from engines, sir.
KIRK: Impulse power then. Blast us out of this orbit.
BRENT: Impulse engines also dead, sir.
SPOCK: Engine room, we need power!
To assume that Scotty gave them impulse power is not consistent with the episode's dialogue. Scotty gave them back only helm control that can stabilise the orbit - that's consistent
They never did say how they were going to heat up the matter-antimatter fuel (other than they couldn't do it fast enough.) If they had fusion power then I would think they could've blasted out of orbit or at least moved further up to buy more time with the impulse engines. Since they did not I don't think the impulse fusion engines were available or could be turned on in time or were in some way dependent on main power to start up.