It would be better to target different variation of keywords than only 1 keywords. Because when you target 2 or more keywords you still have possibility to have rankings on the other keywords.
That depends on the keyword. If you are targeting a difficult keyword ,better target its alternatives too.
It is easy to target one keyword compared multiple keywords. I suggest go with 2 keywords, 1 under moderate competitive and the other low competitive. Once, you able to get fist page for low competitive, change it with new keyword. Follow the above steps until you reach top spot for moderate competition keyword. I don't suggest you to even try for high competitive keywords if your website is less than 2 year old with no link building activity.
No it wont make negative impact infect its really good to focus on one Keyword and optimized it. But the keyword should be most search able keyword or most popular conversion rate ..
No, it won't create any negative effect. But if you target two or more keywords you can get better traffic results.
Why would you want to target only one keyword? You are more likely to bring more traffic by mixing it up a bit.
depends on the keyword Could be quite a competitive word...pick some tails/variations of the keyword.