Big data case study or use case example
News outlets tend to use "Big Data" pretty loosely.  Vendors usually provide case studies surrounding their specific products.  There aren't a lot out there for open source implementations, but they do get mentioned.  For instance, Apache isn't going to spend a lot of time building a case study on hadoop, but vendors like Cloudera and Hortonworks probably will.

Here's an example case study from Cloudera in the finance sector.

Quoting the study:


  One major global financial services conglomerate uses Cloudera and Datameer to help
  identify rogue trading activity. Teams within the firm’s asset management group are
  performing ad hoc analysis on daily feeds of price, position, and order information. Having
  ad hoc analysis to all of the detailed data allows the group to detect anomalies across
  certain asset classes and identify suspicious behavior. Users previously relied solely on
  desktop spreadsheet tools. Now, with Datameer and Cloudera, users have a powerful
  platform that allows them to sift through more data more quickly and avert potential
  losses before they begin.


.


  A leading retail bank is using Cloudera and Datameer to validate data accuracy and quality
  as required by the Dodd-Frank Act and other regulations. Integrating loan and branch data
  as well as wealth management data, the bank’s data quality initiative is responsible for
  ensuring that every record is accurate. The process includes subjectingthe data to over
  50 data sanity and quality checks. The results of those checks are trended over time to
  ensure that the tolerances for data corruption and data domains aren’t changing adversely
  and that the risk profiles being reported to investors and regulatory agencies are prudent
  and in compliance with regulatory requirements. The results are reported through a data
  quality dashboard to the Chief Risk Officer and Chief Financial Officer, who are ultimately
  responsible for ensuring the accuracy of regulatory compliance reporting as well as
  earnings forecasts to investors


I didn't see any other finance related studies at Cloudera, but I didn't search very hard.  You can have a look at their library here.

Also, Hortonworks has a case study on Trading Strategies where they saw a 20% decrease in the time it took to develop a strategy by leveraging K-means, Hadoop, and R.





These don't answer all of your questions.  I'm pretty sure both of these studies covered most of them. I don't see anything about tool selection specifically.  I imagine sales reps had a lot to do with getting the overall product in the door, but the data scientists themselves leveraged the tools they were most comfortable with.  I don't have a lot of insight into that area in the big data space.