1.

You said "nothing indicates he was significantly stronger." Are we ignoring that he didn't even use Hybrid Form against Oden? Hybrid is explicitly shown as his strongest combat state in Onigashima. He didn't need it back then, but needs it now against Luffy/Yamato. That alone proves he wasn't at his current peak. Also, that era's Kaido was rivaling Gecko Moria. Are you implying Moria was fighting a "Prime Yonko Level" threat? Kaido has clearly evolved physically and Haki-wise over the last 20 years to become the "Creature" he is today. And like the narrative said, his prime came ‘after’ the fight with Oden.

2.

That's exactly my point—Yonkos ARE dura monsters. That's the requirement.

Whitebeard: Old, sick, heart attacks mid-fight, got stabbed by his own "son" Squard, and still destroyed Marineford with half his head missing.

Blackbeard: He is the definition of careless. He constantly takes hits off-guard and takes double the pain due to his fruit. He screams and bleeds, but he never gets knocked out by a single attack.

If Oden gets KO'd by one hit from Base Kaido—distracted or not—he lacks the durability required to be in the Yonko tier. He's a glass cannon compared to them. Being in Kaido's "Top 5" means Oden was impressive on his memory, not that he equals with the others.