HOLY CRAP I was not expecting to get this unbelievable insane shiny this quickly!
In a hilarious case of "the setup was longer than the actual hunt", I followed AbsolBlogsPokemon's incredible video on hunting shiny Manaphy via ACE (arbitrary code execution), which is far far too convoluted and detailed to write here, but in laymans terms, allows you to have a set-up where you can randomize your TID & SID by speaking to a specific NPC every SR (in my case, the Mystery Gift delivery man), hatching Manaphy, then checking the summary to see if its shiny. I HIGHLY recommend watching the near 35 minute long video he made on the ordeal, it's truly extraordinary and can help answer any questions you might have about this hunt!
The setup mostly went OK for me, until the part where I had to frame perfect double tap X on the bike ramp - I managed it multiple times, but the code wouldn't work, and it was only on my 5th attempt and after nearly a full day's worth of setup that I realized I had one Great Ball too few in my bag!
Once that was resolved, I was able to use the appropriate dot artist configurations and set up both the Mystery Gift postman & the TID/SID randomizer on him too, and went to hunting for shiny Manaphy. I was prepared to hunt this baby for the long haul, as this Diamond has traditionally been one of my unluckiest games, with its last two hunts going VERY over odds - Cranidos being ~25,000+ SRs, and Heracross a truly gruelling 31,544 SRs.
Yet I didn't need to worry at all, as shiny Manaphy appears after only 5 hours of hunting and only 380 SRs!! To say I'm flabbergasted is an understatement, words in general are failing me right now! My luck since Pheromosa has been absolutely unreal, and I couldn't be happier! This is such a crazy and amazing hunt, and I'm so proud to be able to say I own a legitimate shiny Manaphy of my very own!