Anything can be designated as a target. There are no limitations. Even though the spinal mount is hypothetically very limited in its angle of fire, the game permits the ship to shoot at any opposing target without limitations - and most importantly without impairing its own agility. This tells us that the ship does not spend the entire turn training its spinal mount on the target; it doesn't even spend long enough doing that to impair its evasive maneuvering. Ergo, the time spent actually pointing the spinal mount at the target is a very small percentage of the turn.
Blind spot? Book 2 is running a hundred kilometers to the millimeter. There are no blind spots at those ranges, not in space.
How so?? The missile leaves the tube, orients, activates drive. I see no loss of fuel at all, and the loss of time is on the order of a second or two. These missiles are covering 30,000 kilometers in a turn; I don't think 30 meters is even noticeable on that scale.
The ship/boat launches one missile from a given launcher in a thousand second turn. If the missile was responsible for locking and pursuing from the tube, as it were, the ship could launch as many as it could shove out that tube in that time and basically machine-gun the opponent with missiles - or at the very least put three missiles in space per turn, since that's the rated capacity. That is not the image the game offers: the game has one missile being launched per tube per turn, which in my mind suggests that the missile is depending heavily on the launching ship's sensor feeds for the early phase of its attack. Which makes sense given the size of the missile's sensors, the size of the ship's/boat's sensors, and the tens of thousands of kilometer ranges we typically deal with.
There are no firing arcs in Traveller, not in Book 2, not in Book 5, not in MegaTraveller. There is nothing that limits a boat from firing missiles or lasers at a target in front, behind or to the side of its flight path. One cannot create rules from artwork, and that passing mention that says the weapons are "probably" rigid doesn't occur until Book-5, wherein we are also told the boat has 100% of its batteries bearing. Your referee was wrong; it happens sometimes.