"be that as it may" starts with a subjunctive " be that", which can be paraphrased as "if that is". The next phrase is a fragment, paraphrased as "as it may be" ('as' is a conjunction here, meaning 'like'). The copulas in each phrase are empty, so the 'be' is existential (the complement is its existence, or truth; therefore a full paraphrase would be "if that is true, as it may be true". The author is trying to explain something without admitting that what was stated is true.