itch.io

It's an html file just open it on your chromebook (yes i know this is 7 months late)

They aren't calling you a scammer, it's just that if people tell you what they are going to make then you'll end up borrowing from that idea or even copying it even if you don't mean to. the point of the jam is to try to make something unique

I don't see why not. There's no rules against working in groups. Just put his name or username somewhere (at least on the itch page write "art by my brother")

I guess technically you could but you need to be able to submit the game to Itch

The macOS/X package on Itch.io does not contain an exectuable when installed through the launcher. Itch.io automatically extracts the pkg(which is a modifed zip file), which produces three non-executable files (Bom, PackageInfo, and Payload). This can be fixed by downloading it directly from the browser (outside of the Itch.io software). After downloading, open the .pkg as a normal file. Then the software will install as normal.