Parents planning to Visit PR holder son in NZ

My brother is a PR holder in NZ and my parents are planning to give him a short visit. I have done some research on my own about the visa options and apparently there are 2 options in this case:

1. Visitor Visa - valid for 18 months out of which 9 months you can stay in NZ
2. Parents/GParents Visa - valid for 36 months out of which 18 months you can stay in NZ

The information is so confusing that I am not able to refine the data. Can anyone please give some inputs on the below queries:

1. My parents will be visiting my brother not more than 3-4 weeks and perhaps not go again for next couple of years. What is the best & easiest visa option in this case?
2. For Parent/GParent visa, medical and police certificate is needed. Is it also needed for normal visitor visa if stay is not more than 3-4 weeks?
3. How to show immigration officer that one intends to stay for just 3-4 weeks and will come back from NZ after that?
4. What is the difference between character certificate and police certificate? As per my knowledge, both are same and only police certificate will suffice but need input on this as well.
5. Parent/GParent visa needs sponsorship form to be submitted along with application. Is it mandatory for my brother to personally fill-in the details and duly sign the document and send back to me so that I can apply for my parents?

Kindly illuminate.
Thank you.

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