What about finding him in a state of shock aboard the Constellation?
That's somewhat akin to the police flagging down a driver who's too tired to stay on the road. Once you get him to halt and come out of the vehicle, the driver will be wide awake! The moment of physical shock clearly passed before McCoy was able to take authoritative readings, and in any case was ancient history when the question of relieving the Commodore came up.
Although if it came from outside the galaxy and was fuelled by planetary debris what did it exist on between the celestial island galaxies?
It's pretty weird that Spock could claim with such authority that the beast did come from outside the galaxy. The only way to find out whether the DDM had savaged a given star system was to go into that system and take readings there. Spock's ship surveyed, what, half a dozen systems? Even if these formed a beeline through space (the odds against any half a dozen neighboring or semi-neighboring systems being in line, or even roughly in line, are pretty much zero!), how could Spock claim that the starting point of the DDM was outside the galaxy? Any beeline would
eventually point out of the galaxy, just like all roads lead to Rome, but Spock has zero data on the travels of the DDM beyond the immediate stretch. If the beast feeds on planets, it would be highly unlikely
not to make a sharp turn when encountering the edge of planet-rich space!
Also, if Spock has observed a beeline, then the odds of that pointing directly at a Federation star system are pretty much zero again, unless the Federation encompasses the entire Milky Way or nearly so. The latter might have been assumed in some of TOS, but probably not by most writers; it's clearly not true in the wider context.
If, OTOH, Spock has observed a zigzag course that makes the best possible advantage of local food resources, then any attempt at establishing a point or line of origin or a point or line of destination are pretentious at best.
Most of our heroes' analysis of the DDM is speculation anyway. Does it really eat planets for food? For all we know, splitting and digesting planets is hard work that starves the beast, after which it has to recuperate and gather energy from somewhere else altogether. Is it a vengeance or MAD weapon from an ancient cold war that unfortunately went hot? For all we know, it's a berserker instead, a weapon designed specifically to preemptively destroy everybody else so that the masters can sleep their nights in peace. Or a terraforming machine gone mad. Does it have hostile intent, in the criminal or military sense? Everything speaks against that - it doesn't even chase starships, beyond what it takes to shoo them away from the de-construction zone...
In this sense, it's Trek at its weakest, or our heroes at their weakest: science doesn't carry the day, and blind aggression is the solution of both the heroes and the villains through and through.
Regarding the effects, the use of "phaser beams" in both the original and the redone version is appalling. In the original material, the beams are cartoony, sketchily drawn, and employed in a dull, two-dimensional manner; the one blast from the
Constellation is the saving grace there. In the remastering, the way the thin beams just ride along the side of the beast without attempting any sort of dwelling (supposedly a key tactic if you want penetration) makes the sequences look like a primitive computer game, which in some ways is even worse than being cartoony... And while three-dimensionality finally makes an appearance, it's at the cost of scale.

IMHO, that is.
Timo Saloniemi