Eyewitness testimony is important in trials, but people certainly do recall history differently.  That is why in order to find a person guilty in a trial, the jury needs to decide the verdict based on a preponderanceof the evidence, not on any one thing.  That is also why it is relatively easy to call someone's testimony into question during cross examination, by challenging their memory as a whole, or their recollection of specific facts.

That is also why, in looking back at history, one must take into account a number of perspectives in order to be able to make a judgment about historical events.  Unfortunately, in many cases eyewitness accounts are the only history we have to work with.

As to the first part of your question, the way a question is phrased can definitely alter testimony. This is why attornies questions are often objected to in court as leading the witness. Multiple scientific studies have shown that human beings are not always reliable at remembering the details of what they have seen. They tend to remember things differently, as well, as time passes and they are influenced by what they have heard from other people's impressions of the scene as well. Also, witnesses sometimes lie about what they have seen, either intentioally (to get attention or some other form of gain) or unintetionally. Here is a good analysis of the reliability of eye-witness memory:

http://writ.news.findlaw.com/dorf/20010516.html

As to the use of testimony elicited under hyponosis, the Supreme court banned it as unreliable in 2007. See this article for further information:

http://www.rickross.com/reference/false_memories/fsm117.html