Akainu wasn't "really hurting", I don't know why people say that. He was immobilized for a moment but that does not in anyway mean significant damage has been dealt. Large blows to the abdomen tend to leave people winded. The liver blow is a key shot in boxing for that reason entirely, actually. On top of that, Akainu was just fine and lacked even a drop of blood afterwards. He wasn't noticably injured at all until shredding all of WB's commanders. So I have no idea in what universe was Akainu severely injured after WB's quakes for you to say Mihawk would be. Next, it doesn't matter if Mihawk is Akainu or not. As a top tier they should have similar durability. Him and Kuzan seemed equally injured for the most part after him and Akainu's bout. Kuzan who fought Jozu and was injured by him (regardless of how little). Prior to that one could say Kuzan had no durability feats in comparison to Akainu. In fact, he was injured by a WB commander the same bunch who couldn't even bypass his intangibility. Yet what do you know, they were both said to have had near fatal injuries after their duel. This leads me to believe that all top tiers have relatively similar durability (excluding Kaido).
Now, what on earth are you even on about? WB's first Heart attack was completely random. As well as his second. His health is steadily declining and without his life support he is constant danger of these heart attacks. He wouldn't magically not have them unless injured, that's not how sickness works. Any fight with Mihawk would last for at least a day. How long would WB hold out? After less than 5 minutes of accumulated battle with Akainu he was on his deathbed, really. WB lacks stamina and proper health. And whoa, your tier system is something else. How can Roger, WB, Akainu, and Garp be the only top tiers? What? What is Shanks? Or Kaido? Didn't Kuzan (the portrayed weakest Admiral) nearly kill Akainu in a ten day duel? Lmfao, if the Yonku aren't top tier then... Lol.
And please, you're purposely misinterpreting that. He was testing the "true" distance between him and the strongest man in the world. He was seemingly standing right there but how far away was he really? You're seeing it as testing 1 on 1 strength when that wasn't even the gist of the metaphor. Kizaru also got a taste of the "true distance" in between them and WB when he also attacked and was intercepted by Marco. Is Kizaru losing to old, sick WB without extreme diff? No. Mihawk would take limbs from this man just as Akainu would have if not for PIS.