What does it mean when someone says in a debate "with all due respect, if there is any due" and would it be considered a little disrespectful or very disrespectful? Also these days is the phrase "with due respect" used as a respectful way to disagree or a subtle disrespect?
That's very disrespectful.
The basic idiom here is "with all due respect." It is a polite tag that precedes some kind of disagreement. According to the Oxford English Dictionary, the expression has been around since the 17th century. It is often preceded or followed by someone's name, formal title, and/or praise, as with this example from CBS News:
With all due respect to the immense talents of WWE champion Daniel Bryan and AJ Styles, with Royal Rumble just a few weeks away now, the featured title on the SmackDown Live brand is unquestionably the SmackDown women's championship.
The phrase can be used sincerely or sarcastically, and I've encountered it regularly with either meaning. In any case, it acts as an indicator of politeness.
In your debate, someone has effectively short-circuited the idiom by raising the question of whether any respect is due. Rhetorically, it is a personal attack, suggesting that you don't have the ethos or standing to garner respect. It is a rude and clumsy rhetorical move.