Remember that consulting cases are essentially a tool to test certain aspect of your personality and have a "quick access" to how your reason. As a result, they are simplified version of real business problem and you can expect simplifications, not "totally-sound" technical assumptions and numbers. Most of the time numbers are also made "simpler" to allow for "on-the-spot" calculation.
Do not worry about this too much, but concentrate on showing business logics - if you really have a business doubt on a number / assumption - point it out to the interviewer, but remember not to appear as a "nerd" of numbers - it is still a case interview...
Same as before. Most of the time solutions of consulting cases (especially on casebooks from MBA etc..) represent the whole "logical" spectrum of answers fore each part of the case. Some of them can be somehow "simplistic", but do not worry: they are meant to make you learn how to think logically, being MECE and explore the whole spectrum of the business problem.
Hope this helps.