I learned the grammatical structure of の for questioning (and explaining) things a while ago, but the exact meaning and which gender uses it has kind of confused me.
Here are a few varied sentence I'd first like to confirm whether my understanding of the meaning is correct (I've translated them based on the feeling I'm get when I hear them):
暇があるの？ - You have free time? (I thought you didn't)
暇がないの？ - You don't have free time? (I thought you did)
暇があるんじゃない？ - (But surely) You have free time don't you? (Confirming as if you know they don't) - do I need a の at the end?
暇がないんじゃない？ - (But surely) You don't have free time don't you? (Confirming as if you know they do) - do I need a の at the end?
暇があるんじゃないの？ not used because it doesn't make sense / sounds weird? And then should you add
か to the end because it sounds a little feminine or replace
のですか？ to be more formal?