For 1.see this stackoverflow question
Basically the most common formula used, haversine formula assumes the earth is a perfect sphere which is wrong. For better accuracy Vincentys formulae should be used. I guess it depends how accurate you need to be (does it matter to falsely include/exclude?) and I don’t have personal experience with this so I suggest to google what is better or wait for more answers.
For 3:
I know you can do that with R or python for example using plotly and you can use python/R in knime. However if you already have tableau professional and it can do that it is probably the easier way to go. Knime does have nodes to send data to tableau.