Q1: Analyze why the most severe and expensive errors may be easier to fix in the early stages of the system development cycle rather than after implementation.
A: The most important and difficult determination in any product development is to correctly identify the premise(s) necessary to the product's development.
Product development usually tracks through separate design and production segments. When a creative designer imagines and even draws a conceptual design, he/she is operating in a two-dimensional space where the laws of physics do not apply. An aircraft, for example, may look spectacular in the designer's mind, and even in an illustration -- but, if it cannot fly in the physical world, then nothing else matters.
The above may seem an extreme example, but it actually applies to every part of early product development. If a product seems terrific, but getting it to market will require so much time and money that the business organization will go bankrupt before the product is in the consumer's hands, then the product is actually of no value at all.
Early discovery of roadblocks to end-user delivery can avoid mistakes which could literally make the product unmarketable. Once a product is in the production phase, if an insurmountable roadblock appears, it's usually too late to do anything other than to cancel the project entirely, because the design budget is already expended, and the cost of going "back to the drawing board" may render the ultimate product price too high to be marketable to the target consumer.