To what extent does the truth of what we say depend on the literal meaning and how much of what we mean depend on context?

I think that what you are examining here is the branch of
ethical study called deontology.  Deontological ethics argues that individual actions
have to adhere to prescribed rules and adherence to rules of conduct.  This is where the
assertion of "literal meaning" is present.  For example, if lying is wrong based on a
code of conduct, then it is wrong in all circumstances when it is placed.  Context or
contingency is not something that needs to be examined.  This would mean, then, that one
does not argue, "What if a person was in a situation where they had to lie?" 
Deontological ethics argues that if there is a code of conduct to be followed, then the
ethical right thing to do is to uphold this code.  A consequentialist theory of ethical
conduct is where context and the results of one's actions are examined.  In this
setting, ethical action is dependent on context.