"Hi yn" sounds very weird though. Not something I think a native would say, even if it's grammatically acceptable (which I'm not sure it even is...)
Yes, ...hi'n... is usual here and should be appearing about half the time in this version of the course. The unusual ...hi yn... has been deleted from the future update of the course - we cannot amend it in this version at present.
"oren golau yw ei llygaid hi" was marked wrong
That is an emphatic pattern - no real need for it here.