I feel it's not crazy to infer that he can produce DJ without a mandatory requirement for a modification. Luffy can produce fire too and despite being a DF user that alone doesn't justify how casual that occurs. Furthermore there are other characters capable to generate elementary attacks despite possessing no peculiar DF power allowing so.
I doubt this would work. Judge would need access to the lineage factor of these races and as we’ve seen from Caesar and the numbers making hybrids of other races was not a well established science back when Sanji was born.
I just like the idea that Sanjis fires come from his burning emotion and passion his mother died to allow him to keep. It’s a beautiful story and doesn’t need much more digging.
Indeed. One of the very few interesting layers the Vinsmoke plot stressed was Sanji being an emotional person who got his powers from that, obviously represented by his fire. The idea of a cook who works with fire, isn't afraid of it and grows stronger the more spicy his feelings become perfectly contrasts the scientific coldness of his biological family; which is exactly why the Raid Suit was a bad idea and why Sanji doesn't need to awaken whatever DNA modifications. That's the kind of idea from people who lost their ability to appreciate symbolism and narrative.