1. Fear that a major vulnerability is found in a commonly used software package that puts multiple major banks and e-commerce sites at risk
2. Fear that major vulnerabilities are found in multiple, widely used software packages that lead to market downturn as IT company stocks crash.
Probably others as well. Sounds more like a brief on worst-case scenarios that may happen and how they would effect the US banking sector. This is an important mid-year election this year too, so any big economic shock would be bad for the GOP.
No, traditional western finance isn't like a unified database, but rather many controlled by different parties in different countries. In theory you could write it back, but it's closer to bittorrent - if someone downloads a song from you then you can't just take it back.
Some stuff can be voided and there are clearing house mechanisms but if money moves fast enough across enough borders then getting it back is near impossible