Well if what happens is a few fusion reactors creating a bubble which lightens the ship's mass causing it to have unprecedented rates of acceleration, what are the giant glowing red ports on the back of the saucer for?
Those are the impulse engines. Here
is a blueprint - it's not the Abrams version, but I can't imagine that something like that would be any different. (I can't seem to find blueprints of the Abramsprise online.)
I don't know if increasing the mass of something in space causes deceleration per the theroy, maybe it does. Do suns slow their respective galactic speed as they go to red giants?
If they do slow down, it's not because they've gained mass - if anything, they've lost it. Some red giants - typically toward the end of their red giant phase - blow off the outer parts of their atmosphere.
Their rotation would slow down due to the conservation of angular momentum, but not their actual motion, unless they were acted upon by an outside gravitational force.
(Disclaimer: I'm not an astronomer or physicist - just an enthusiast. I do have a math degree, but with a very small number of exceptions, I haven't had to do any higher mathematics in a little over 20 years.)