Here's the thing though, BB doesn't put his opponents away with strikes from the outside — this is what beats Mark Hunt – whereas BB does his damage from the top position and an opportunistic scrambles. Mark Hunt isn't going to be looking for the takedown and this will almost certainly be a kick boxing match inside MMA rules. Like I said, what beats Hunto has been work from the outside. Faster opponents who are able to explode or use their diversity to exploit his flat footed kickboxing approach. From the JDS wheelkick to the speed of Reem is what beats the Aussie. For all intents and purposes, TBB it's not this guy.
But i argue that its that same type of fighter that would give BB a lot of trouble. Look at what Hapa was able to do to BB before ultimately being finished by him. Hunt isn't that guy. He's turned into a plodder himself. I believe this fight is going to turn into a fire fight at some point, and while BB lacks skill and technique, I trust his chin more here. And while, yes, BB's finishes can come on the ground, I don't think it's far fetched to believe he packs some power in his punches while standing as well
What's more, having your chin "cracked" is extremely unscientific in the way that it has been sold ass fucked in modern combat sport lexicon. For instance, Andrei Arlovski was at a point where a stiff breeze was putting him to sleep. He had lost four straight bouts including getting smoked by a far less talented Brett Rodgers. However, after making a few defensive adjustments and tightening up his vulnerabilities, he fixed those liabilities and put himself back in the UFC and in 2015-2016 was aiming for a title shot.
The difference, i argue, is more of him fighter lesser opponents. After the mentioned 3 KO losses to Fedor, Silva, Rogers, and Sergei, he fought the likes of Ray Lopez, Travis Fulton, Devin Cole, Mike Hayes, and Mike Kyle before coming back to the UFC. Yeah, he lasted against Rumble but IIRC wasn't that fight mostly a clinch battle? (correct me if i'm wrong- all I remember was AA's broken jaw in that fight). Sure enough, what happens when he comes back into the UFC? 3 more KO losses and he was dropped by Browne as well.
The same is true with Struve. He was getting snuffed every time you turned around. JDS, Hunt, Reem, and Hapa were all handing out free naps to "Skyscraper" and he appeared ready for retirement and subsequent brain damage. However, he has since demonstrated an ability to eat potent shots from cannon fire launchers like the champ Miocic along with other bomb dropper's like LavarJohnson and Pat Barry atc.
You're jumping around here- all of those fights were mixed together. He didn't go through a stretch of getting KOed then turn it around afterwards. This was the order of the fights you mentioned, going from earliest to most recent
JDS (KO Loss)
Browne (KO loss)
Lavar Johnson
Pat Barry
Miocic
Hunt (KO loss)
Overeem (KO loss)
Also, Struve isn't 43. Look at Hendo, the guy had a chin made of diamond but his chin started showing signs of weakness after he hit his 40s
Then there's Reem. After getting slapped by Travis Browne, Bigfoot Silva, and Rothwell, he remarkably turned things around and withstood bombs from kjnocckout artist like JDS, Arlovski, Hunt, Roy, and Struve.
LOL Struve landed like one punch in the fight. Overeem's defense has always been decent, its always his damn cardio that fails him and leads to him getting KOed (see: Browne, Bigfoot). He was able to avoid damage against Big Country because he grappled him for the majority of the fight and even then he was hurt in the final round by the overhand right
Basically, you have to be careful throwing around the catchall phrase "a guy with a cracked chin" because fighters can go through a period getting hurt before tightening up the vulnerabilities and then finding themselves able to withstand a barrage of virtual Tomahawk missiles landed on top of their head.
And to your last point, I really dont think Hunt is learning new techniques to tighten his defense at the age of 43. At this point in his career, he is what he is.