I think you're stressing out about perfection a little too much.
Yeah, okay, maybe the coins, theoretically, should have shot out at a vector perpendicular to a line connecting any two competing coinshots (or otherwise, as noted) because of their component parts, but where's the fun in that? The push/pull ability is most likely not a one-dimension vector, but has some facial area--that is finite--over which it acts. Is the force equal across the face? Or does it drop off to some degree as you reach its edges? Can a coinshot play with the distribution of power across the facial area? How about multiple pushes/pulls acting on the same object? Does it only work center-of-mass to center-of-mass? The answer seems to be that yes multiple push/pulls can be used on a single object as per the scene with the spinning bar from (I believe) The Final Empire. In that case, is it coinshot's center-of-mass to a point on a metallic object? Why not just push the coin ten times then before the other guy can push more than once?
I think there are lots of questions we could ask about the magic system that, if answered, would ultimately bog down the story in detail and mess. Brandon was an English major, as far as I know. And even though he tries his best to build things according to modern physics (as noted by Ookla), he's probably not going to get into the minute detail that a physics major would. Granted, this might bother someone who IS a physics major, because they're plotting vectors and equations are tumbling from their head (hey, I admit, this happens to me often. I do after all rank up there with the geekiest of the geeks), but it should probably just be taken at face value and seen as entertainment. That's what it ultimately is after all.
What's my big beef, you ask? Well, since you asked: the ash-mounts. It's been noted (by Ookla also, I think, in another posting) that the reason why the Lord Ruler made the ashmounts was to block the suns rays after their planet had been moved too close to it (instead of just moving it again). So, the ash fills the sky over the populated areas and keeps everyone from getting charred by a too-close sun. Problems? How to balance the amount of ash in the air (from an admittedly natural source) so that enough sunlight gets through to keep a day cycle and allow plants to grow, but blocks enough of the suns rays to keep everyone from baking to a crisp. Or there's also my personal favorite, finding some way to keep everyone from getting sick after inhaling all that ash. Inhaled particulate matter would tend to make everyone sick and die very early and nasty deaths. That's the big one for me, anyhow.
Problems? Yeah, probably.
Fun? You bet.
Enough fun to ignore the "physical" problems? I think that for most, the answer is also: yes.