Without a facing rule then the whole fixed mount paradigm is ridiculous.
Ship A is fleeing towards the 100D limit, ship B is in pursuit, both have fixed mounts.
Ship A, moving directly away from ship B can still fire is fixed mounts at ship B.
Ship A decides to fight so swings round and now accelerates towards ship B, somehow the fixed mounts magically now point in that direction.
Ship B decides to run back from whence it came and so swings around so it is now facing in the opposite direction, and tet once agin the fixed mounts can magically still hit.
Fixed mounts need a facing rule.
As to dogfighting, those rules are broken thanks to the change of time scale, they are ridiculous from an established in setting Newtonian movement paradigm, the only place for them is in the cartoon pew pew cinematic physics of a Star Wars like setting, which Traveller in all its nearly 50 years, has not been... until Mongoose decided to make it so.
By all means have dogfighting rules, but they should not be applied to the Third Imperium setting if you want to maintain "canon consistency" (nearly a fortnight and still hilarious)