I will be flying to japan and entering china from South Korea. Can I enter China from a sea-port? and re-enter from HK if I hold single entry visa?

i do not have a specific date for my entry to china, is this ok?
can i enter from a sea-port?
i do not have a specific date for leaving, approx.2 weeks after entry, is this ok?
can i enter hong kong and re-enter china on a single entry visa?