I'd like to help and I'm going to put my mind to it. I haven't written any fanfic yet but I'm a writer and I love trying to figure out solutions to plot problems.
Just one question: I'm not sure I understand why exactly you want to crop out the Culverton story. You say it's unnecessary but then you admit that you need something that brings the characters to the same place (so it is necessary at least in that sense). So I'd like to understand better what is exactly that you don't like about the Culverton plot and in which way should the new event be different to the Culverton story.
Also, maybe this fits better in the Fan Fic subforum? I guess the mods will take care of it anyway

Just moved the thread here. Good luck with your fic. 

Ok, now I think I got it better. I'll think about it! I can't promise anything but I'll do my best!  

Oh my gosh, Elemental. I am sorry times a thousand. Somehow my notification for this got drowned in an overflow. My deepest apologies--you took time and effort to help me and I disgraced you by my lack of response and gratitude. Please know that I am endless grateful and appreciative for your assistance and brain power. Endlessly.

I think you're so right about the challenges of getting around John's being compelled proving to be a problem. And you've given awesome and inspiring suggestions. After contemplating your thoughts, I realized that if I am to do this, it might be necessary to shift my focus from Sherlock getting John into the flat, to a more, what could John need to from Sherlock that might make him more easily swayed by an (whether it be a scenario Sherlock concatenated, much like in TLD) outside occurrence that might cause him to be in Sherlock's flat more--and eventually permanently. 

To answer a few of your questions, it is indeed a Johnlock fanfiction--however, it is a parentlock based fanfiction and the Johnlock is an eventual realization that doesn't really start to come into play until later on. 

As for becoming more specific with the possibilities, this is what I have scribbled down so far:

After Mary is killed, both John and Sherlock start a downward spiral in their own respective ways. Sherlock is riddled with guilt and shock, and is slowly getting eaten alive by John's absence and rejection.  John is also struggling. He can't cope, can't go a day without constantly hallucinating his dead wife and is quickly falling deeper and deeper into alcoholism. He becomes reclusive, and all the while Rosie is nearly permanently living with friends. A few days after Mary's death, Sherlock receives the "save John Watson" CD from Mary. ( I will probably tweak the CD's continence a but to help achieve the following plot) Sherlock sets out to follow Mary's last wish and save not only John, but Rosie. He does this by immediately diving head first into drug abuse, knowing the word will get to John. Then he starts really loosing it. Mrs. Hudson seeks out John and begs him to help Sherlock, tells John of Sherlock's digression. John, who is numb and bitter, refuses. But the thought is now stuck in his head. (just as Sher plans) Finally one night Mycroft calls John and explains that an intoxicated Sherlock has left his flat and is reeking torment to the London streets, endangering himself, and then managed to escape Mycroft's protective radar. John refuses again. But later on ( with the help of Dead Mary) John finds himself out looking for Sherlock, (just as Holmes planned) only to find that Sherlock has left him a trail, obviously knowing that John would come. John finds Sherlock in a deplorable state (much like TLD) and Sherlock explains that he needs John to help him with a case blah blah blah ( basically exactly like TLD when Mrs.Hudson brings Sherlock to John and Sherlock, just as Sherlock knew she would, and then Sherlock explains about Culverton, and explains he is unable to bring the man down because of his deplorable state. He needs John. And John needs an escape, he needs a case, he needs his adrenaline junkie high.) Except instead of Culverton, I was thinking I could just invent some other scenario that would involve the imminent danger of innocent people (to appeal to John's heart) and ask John to help save him. Then Blah blah blah, they go save the day, basically as things went down with Cuverton. I haven't figured out what specifically yet ( so if you have any ideas? I was thinking something to do with an imminent bombing...but that plot already kinda happened. I was also thinking that possibly it would be something Sherlock completely made up and orchestrated for the soul purpose of giving John something to save/do with him. Like Sherlock paid actors and stuff, like the whole Moritary isn't real stint, except make it REAL. If that makes sense? Sherlock stages a bad guy. There. And then that can come up later and make a big stink. ) The blah blah blah they save the day and in the process, revert to their own ways and save each other, then we end back up at the flat and the whole Hug Scene from TLD happens, and then somehow I'll use John's emotions and inability to cope and Sherlock's risky drug addiction and need for surveillance will come together and somehow get John in the flat--Just as Sherlock has planned all along. 

That was so long and so rambling I am so sorry UGH. You didn't need all that, but it just kinda happened--actually just typing all that out really helped me figure out stuff...wow.