Key word is FORM. his FORM, not his nature. He took a FORM far from human.

"We once had a master. He created us long, long ago. But he was born with a fatal flaw. He resented those who had what he lacked, and became fully mired in hatred. Eventually, he drove himself mad. It was at the peak of this madness that he conjured up we strange creatures."

"Our master was a tragically lonely soul. Eventually, his solitude eroded his very reason. In the end, he never understood… …what it was that he truly lacked."

We know what seath lacked, immortality. he attains this in DS1 through his crystal experiment. This line contradicts that. Spiders also do not possess immortality, they have NOTHING to do with it. And Seath is dead. He was killed when you shattered his immortality crystal and he DIED. His soul is reduced to REMNANTS, by the Eons, which means he wouldn't HAVE a form. He CAN'T be the spider because he is no longer.

The bug is the sinner, because relighting the flame turns you into a bug. the evidence is in dark souls 1.

The prisoner is an old man because he looks like an old man. the concept art is an old man and the 3d model is an old man. you can clearly see it on the pc version with the resolution up.

Yeah, the actual lord souls descriptions contradict the idea that the old bosses would still be "corporeal" in any way. There's other explanations in other items and lines of dialog that give you a story with NEW characters that just TREND after the old, due to possessing influential souls.

In order for seath to be alive and a spider you have to IGNORE the soul description that says that eons of time reduced him to these remnants. If a theory has to ignore item descriptions and lines of dialog to work, then it's probably an incorrect answer. The simpler solution is that it's new boss characters. Nothing contradicts that. You don't have to "make stuff up" or jump through hoops to get to that answer.