What is the use of saying 'mimi' when 'ni-' is already conjugated in the verb? Or is this used to stress the fact that -- I -- am doing this?
the ni in ninapanda already implies that the action is done by me. The addition of mimi at the beginning is for emphasis as you have already correctly guessed.
It sounds odd unless for emphasis. I think it would be safe to avoid saying "mimi nina___" in general usage. Also people often drop the "ni" for "napanda" as well as it's implied when you are the one speaking.
IME the Kenyan English translation of this would be "Me, I am climbing/planting". Perhaps it's similar in TZ? In my opinion this is correct English, even if it wouldn't be considered so in UK or USA.
I know that -panda can mean to plant/sow, but I was also under the impression that it can mean to organize or arrange. So should I report "I am organizing" as incorrect? Or am I an idiot?