For question 1… most marketplace creators simply don’t have access to a OSX computer, and so they don’t mark them as compatible because there are some differences in using UE4 under windows when compared with OSX. One example is the shader model - until recently, OSX could not use shader model 5.0, which meant a heap of material features simply didn’t work, or didn’t work as intended, on macs. In practice, most packs that aren’t plugins likely WILL work fine on mac, but because it’s very hard to tell without testing, most sellers don’t want to risk upset customers (which is fair enough).
Question 2… that’s odd. I guess the launcher only shows content that is compatible with your current OS? Never heard of that =/