Bret getting the belt at all was cos Flair had given 3 months notice... he didn't care about Mania 9 cos he knew by then he was not one of Vince's top guys. To be fair to Vince he held his end of their deal up and let him walk and Flair was more than happy to put Hennig over on the way out, but not to stick around 3 more months.
It's ironic that basically Hogan and Warrior can be traced back to every talented guy who could have headlined the company of that era who walked or refused a deal in that era... Bam Bam in 88, Rude and Vader in 90, Flair in 93, DiBiase in 93 for AJPW (where he got hurt) they all got the Hogan and by extension Warrior treatment and all left because of it.
Flair gave notice ?? When, as he was reigning World Champ ???
Flair didn't give any notice. He didn't want to leave. With Savage taking himself off the road and reducing his own schedule the plan in the fall of 1992 was for Flair to regain the belt (which he did) and face off with Ultimate Warrior, eventually with Warrior winning the title, likely at Survivor Series. Those plans got changed after WWE spent a month hyping Flair-Warrior matches (a feud that started due to Flair's involvement in SummerSlam) when Warrior injured Flair, screwing up the Press Slam move, I believe in Phoenix. Flair suffered broken bones in his ear which dislodged and ruined his equilibrium or sense of balance. Flair was off TV for over a month, portrayed as recuperating from injuries. Right before he left (he was headed to Mayo Clinic for treatment, no one knew at the time what was wrong) he agreed to lose the title , WWE didn't want to keep the belt on him with no timetable for his return. However, Warrior was already on thin ice with WWE and injuring Flair was about the last straw. Brett got the belt because 1) Savage didn't want it 2) Flair was hurt and couldn't keep it 3) Vince no longer wanted to put it on Warrior. Hart was just inching into the main event sphere at this time and had been wrestling full time in WWE for almost 7 years by 1992, he was popular, a good choice for the WWE preferred fan favorite champion, and he could wrestle nightly, something WWE wanted (and wasn't sure they would get with Warrior) seeing as how business had been declining since 1990.
Hart didn't get the belt because Flair quit the company, it wasn't until after Flair returned and began wrestling again in November that Vince said he didn't have any main event plans for him (beyond putting over Hart in re matches on the house show circuit, they never had any rematches or a big build up to the title switch due to Flair's injury). Flair had been given the option to leave if he didn't like the creative direction when he first arrived and decided to exercise it (nearly two months after Hart was crowned champ) since WCW offered similar pay with a much reduced work and travel schedule (WWE was running twice as many house shows at this point as WCW was, and Flair had already been in the UK, Germany, and all over Canada main eventing).
Did Hart get the title too soon ?? Maybe, but given the circumstances (remember Warrior completely dropped out of the Main Event scene after injuring Flair, his last appearance a brief interview spot after Flair came back where Flair & Hall beat him up on Sat Nite Main Event, causing the "Inuries" that forced him to pull out of SummerSlam and explain his disappearance) WWE didn't have a lot of options.
Regarding WM 9, the ending was a complete curve ball and surprise and in that way it worked very well. If business had not been down with Hart as champ WWE likely wouldn't have brought Hogan in like they did (booking left a lot to be desired thought, people blame Hart alone for declining business without noticing that business was declining, albeit slower, when Hogan was still around in 91 and 92, plus the company lacked star power to oppose Hart once Flair left, HBK wasn't at that status yet, neither was Nash). My only complaint here is that I felt they should have kept Hogan through SummerSlam, remaining champ, and let him lose there. If the decision was that Hart wasn't worthy of beating Hogan cleanly then having Yoko, a guy who was primed as the company's lead heel, and a clearly physical opponent, getting a screw job win was fine with me, it left open the possibility for a Hogan return and re match down the road (although that didn't transpire) but it still gave Yoko a marquee victory, enhanced his rulebreaker status, and would have made the chase with Hart pursuing him entertaining. Hogan staying for a month and quickly losing to Yoko without much of a feud I felt wasted the momentum of his return and surprise title win.
As for the other matches, It was clear by now that Henning wasn't catching on with fans as a Fan Fave, having him lose to Luger, who was one of the biggest names in the business in the 90s, in Luger's first big singles moment made sense. It would have been nice if Flair had stayed with the company and had his match with Henning here but given that Henning didn't connect with fans the way they had hoped as a Fan Fave even with the win over Flair in Jan I doubt it would have made much difference, it would have been a better quality match though.
I still think it was a good time for Hart to lose and to let a dominant heel take over for a bit (a rarity in the Vince Jr era outside of HHH in the Evolution Days much later). I don't have a problem with Hogan's return other than that it should have lasted much longer, through SummerSlam.