As a South African, will it hurt or help my chances at getting my novel published in the US or UK if it has already been published locally by a respectable publisher?
I'd say no, because publishers buy foreign rights all the time: but this is something you need to ask your agent.
If you don't have an agent, how do you propose to find a publisher in a country foreign to you? If it's by querying agents in those territories, then you do need to hold off on signing up to a local sale because your agent will have more to negotiate if all rights to your work remain intact.
Depends on the region. In general, books are published with the author granting rights per the contract for publication. Provided you don't sign away rights, you can still market them elsewhere. Agents deal with this all the time.Trying to weigh up losing an agent because they don't have the rights to a small region...