Tied directly into the impulse engines, there shouldn't be more than a five point variation in the velocity balance.
Sounds like an issue with the ability to add or subtract transportee momentum all right! I'll buy that rationalization.
Why didn't they just beam down some good old fashioned blankets? If they get duplicated, the more the better.
Nothing they could have beamed down would have been of any help. The landing party would die unless beamed up
- blankets, hot cocoa and whatnot would just delay the inevitable.
That was the whole point of the plot. Everything our heroes did was too little, too late: while the split Kirk prevaricated and procrastinated, doubted and delayed, things escalated so that what had looked like a minor inconvenience gradually became more major and ultimately life-threatening. Beaming down blankets during the first few minutes of the adventure might have been nice, but six hours into the adventure the blankets would have done no good. (Indeed, at that point, the landing party did
have blankets - they wrapped themselves into the canvas of their tent.)
It's the tragedy with any escalating plot: when the end credits roll, the audience goes "they were so stupid
- doing X and Y in the first place would have saved them all the trouble!". But that's unfair, because the heroes "in the first place" had no way of knowing that doing X or Y would be necessary, let alone urgent. There would have been no logic in beaming down blankets at the point of the plot where the transporter still worked but was known to be unable to send down the more complex things needed for ultimate survival. Not unless the heroes knew in advance that the transporters would utterly fail in a few moments!