Try asking OpenFL guys at http://community.openfl.org/

Because it would be the slowest part of rendering. My example may not be the best, because copyPixels in new versions of OpenFL is really slow anyway, so you don't want to use it unless you are ready to do some optimization work on it
Note that I'm talking about native targets, flash target is mostly unchanged AFAIK

Mostly under the hood, but it becomes GPU-accelerated, so you shouldn't, for example, call copyPixels each frame, but use shaders instead to get performance

New version is developed publicly, discussion here: https://github.com/HaxeFlixel/flixel/pull/2068
You should try it already, developers know nothing about your project and don't have it on their computers to test if it would be broken
Since 3.6.1 in legacy mode a lot has changed in OpenFL, and my project got some regressions in rendering

Try Firefox.
Also in Windows, lime automatically runs the local web server and opens it in the browser, does it not work in macOS?

Extension is just a part of filename, the codec to use is specified by flags and signatures in the beginning of the file's contents. Windows and probably HaxeFlixel rely on extensions to distinguish between formats. IrfanView, on the other hand, will show a warning message if extension and format don't correspond each other.

The problem is your PNG star image actually has a BMP file format. You should use drop-down menu in your paint app's save dialog to select the format, and not type the extension manually.

Maybe forgot to call .flush() on FlxSave