i am going to Hong Kong for business trip and will visit factory in China,

1. ML, if you don’t have HK residence permit, I’m afraid it’s not easy for you to apply from there. Of course, if your application is accepted and processed in HK, you can choose the rush service which takes 2 working days.
2. It’s very difficult for you to get multiple entries in HK.
3. No, usually, you cannot get VOA at any port in China. Only few ports provide VOA for nationals from some countries.