Also, her brief hesitations to revile her father and brother might be attacked as attempts to fan the flames of Anna's hatred. Yet she maintains that she is indifferent to Lovelace, and even that he is objectionable. All of these charges, of course, could be answered by the claim that Clarissa always acts according to her extremely high standards of morality, courtesy, and truth. The first is agreement with your observation that the epistolary form seems to insure that we will hear the true thoughts and feelings of the characters and equal agreement with the assertion that Richardson often undermines confidence in their objectivity and consistency. But true thoughts and feelings are hardly ever objective or consistent.
Thoughts can be entirely sincere yet utterly subjective and inconsistent.
I think that is often the case. It seems to me that that is a part of Richardson's accomplishment in Clarissa. These characters ring true because they are all, to some extent or other, self-deceived and deceiving. If read this way then we can expect these characters to be most duplicitous at their earliest appearances. At this point I will bring another book into the discussion.