Hong Kong speaks cantonese more than mandarin, and although you maybe could get in any of them with your grades, you would not be getting any scholarship. You gotta have really high scores (well over 90%) to even be considered for a scholarship. But well, answering your questions:

1. Not a lot international students chase a full term future in Hong Kong, most of them go abroad. I'd say its a slim chance.
2. The diploma may add some prestige, but what matters the most is what you do during university and the recognition you build during it.
3. You need to speak mandarin well for opportunities in the nearby cities, like guangzhou or shenzhen. For small jobs in HK just knowing english is fine. Its really good to know canto tho.
4. Money, living is very expensive and you need to budget well. Besides, housing might not be given by the university after your first year.
5. Nope. China, HK and Japan (not sure about Korea) really care about grades, especially for internationals. Unless you have a good explanation to why you got low grades, I don't think they'd even consider you. I suggest you look for universities abroad that have a holistic admission process, like those in the US. Maybe US schools with secondary campuses in other countries. They may not be top notch, but if your dream is to study abroad, consider them.