I think you may end up having to post your problem in the Mach3 forums. I read the first post you linked to, from 2007, and it looks like the guy never had his problem solved. Surely the situation has gotten better in the intervening five years?
In any case, CV and sharp corners shouldn't be a problem. Because the tool has a radius, the toolpath is going to be a quarter-circle around a corner anyway, and as long as you don't have the feed rate set so high that you whip around that arc, acceleration should be low enough that you won't have a problem. That's geometry.
And here's math:
Let your feed rate be F inches per minute. And let your tool diameter be 1/4", because I know that's what we're using. A quarter-circle of radius 1/8" around the corner will let you make a sharp corner. The acceleration towards the center of that circle is just v^2/r, so 8 * F^2 inches per minute per minute. So if your feed rate is 220 ipm (or 3.67 ips), then your acceleration around that corner is 387,000 inches per minute per minute, or 108 inches per second per second (2.7 m/s^2, or about a third of a g). Is your machine capable of doing that?
If not, you may need to change your feed rate around corners to be slower, which is what the ShopBot does. It's the max feed rate on straight cuts, but it has to slow down around corners (and so it does, making perfect sharp corners).
Now, as to why you're getting rounded corners in some directions and sharp in others... is it possible that the machine is stiffer in one direction than in another? Make some test squares to see.
And finally, I am mystified by the advice to turn arcs into polygons. To me, anyway, arcs would be much easier to accelerate around. Polygons are all sharp angles, which would seem to make the moves less smooth?