We get this question quite a bit. Please allow me to wander a bit. I will get back to your question quickly- I promise.
One of the things I like about SE is that it is a fantastic window into what people are concerned about and what topics are important. Since this topic has come up several times and I answered this question with scant detail a few times, I decided to write a page in full detail on just this topic. I hesitate to link to my own site/pages here, but this page is a direct result of the SE posts. So here goes. (Please forgive me if this is taboo.)
If you want a good perspective on the value of Alexa, I suggest reading: http://www.closetnoc.org/?i=definitive-proof-that-alexa-sucks
You will see that this takes a more valued approach on the topic than most pages of it's ilk. This is because I prove my point using statistical analysis and not just another opinion. You will also see that this is not such an easy question to answer in just a few short and sweet paragraphs though it is a rather simple topic.
But if you want the short answer, here it is: Alexa is of no value at all. None.
This addresses your three points. To address your first point better, it is plain foolishness for a business to hang any investment or performance value on Alexa. Plain and simple. If my page is useful, please refer to it to help explain why Alexa is clouding the performance analysis market. I think I made a fairly clear case that can help. If I can tune this page at all, let me know.
