I noticed a string of variables thrown about concerning atmosphere, but it apparently has been forgotten that incident solar winds significantly affect atmosphere. Being far away from a star, as well as being in orbit around a dimmer star helps, but the real kicker is the existence of a magnetic field. Earth and Venus both are protected from Solar wind by self-generated fields. Titan is also protected, at least by Saturn's (I'm not sure if it has its own field as well) field. I've seen it hypothesized that Mars used to have a significant atmosphere, but then lost its magnetic field, and the atmosphere with it.
Having said that, a planet half the size of Earth would have to be very young (imminently possible, since young is several millions of years) so that any molten core it has remains molten, or subject to powerful tidal forces, such as from a close-orbiting binary planet, or by turning the panet into a moon of a gas giant (remember that the moon was unable to maintain its molten core though it is subject to tidal forces from Earth) (also remember that very large gas giants, such as Jupiter, exude massive i.e. lethal radiation).
I'd say stick with young, it makes sense if this system is in a nebula, and it's simpler, too. And now that I think about it, the magnetic field from a gas giant could easily screw up terrestrial biology, depending on their sensitivity to it. Even if it doesn't answer your original question, it adds a piece of history to the place.
Since I'm here, though, I might as well tackle gracility: animals that can use more strength will develop more strength, and the lower grav will allow them to keep their speed (imagine a lioness hunting down elephants); animals that could use more speed will lose muscle mass, but that won't affect strength (you've already imagined elephants filling a gazelle niche). Here's the kicker, though: it's known that humans lose muscle mass in zero-g, this is because the muscles do not get used as heavily. The same may well hold true in a 1/2-g environment, but if you've specifically seeded/engineered megafauna, then the usual atrophy from transplantation might be ignored. Ultimately, if you can make a halfway-reasonable explanation, you can do what you want and the reader will fill in the gaps, or at least assume the gaps can be filled by some science that's over their head.
My Conlang Site
which pretty much only has Tayéin.
Still under construction, but at least I did some photoshop.