dday......This is going to be long and unsolicited. I hope my words offer some help.
First time posting to you but have been following your story as the nuts and bolts of your situation are similar to mine. I too am divorced from my now reconcilled (x)W.
Our "falling out" was very fast and furious. She basically took off after having just the right (wrong) people to tell her to leave and her ignoring the ones that truely did care for well being. After 6 months of turmoil I filed and got full custody of kids and everything we owned with no contest from her.
We went our seperate ways for about 6 months and she basically ignored the kids during that 6 months. I saw her downward spiral and did what I had to to protect the kids from a tainted view of their mother. I did this knowing that she would someday come out of it and step back up to the plate of motherhood given her upbringing.
So, as she came out of the tailspin of her "great new life" and I had well moved on with my own (sans female attention) she saw what it was she left and began moving back to the family. That took another 6 months and one day she came to me and said she needed her family back. Over the course of the next couple months that plan was set in motion.
I'mm sorry for making the above so long, but it helps with what I have to offer on advice of what has developed with your situation....
Looks like you have an extremely complex situation (aren't they all???). But they really boil down to three or four areas to address. I am guilty myself of trying to fix "everything" all at once. But when I tell myself to break it down to pieces and solve what I can I get much more accomplished.
First area of concern that I see as most important....Your Son. I too have a 12 year old boy. As you know, this is an integral time in his life. What I am seeing by your posts of his behavior is that he is seeking stability!!! So that needs to be your goal and plays into all the other "stuff" you have going on. He is screaming for attention and at 12 he needs to be given that. You and your (x)W are the only ones that have any control over that. My son never lashed out but I took every step I needed to maintain the status quo when my ex and I were apart. Everything from routines to traditions. He still suffered deeply, but I believe the damage was contained by doing that.
So, now ALL THE OTHER STUFF......from what I can see, whether it can be controled or not, it's time to grab them reins and bring things back in check.
Everyone told me to take it VERY slow when my wife and started on our path back together. Until you've been there, you will never understand how difficult that really is. Right?
So I say to you, time to sit down with the wife and a pad of paper and pencil (so you can adjust as needed) and figure out a game plan for YOUR lives. Bring the family back together. No matter how unpopular it may be with FIL or Cousin. Bring stability back to your life. Whether it's in the old house, an apartment, the OM's house, wherever!!! I understand your reservations about each dwelling. But it could be short term until that stability has had time to manifest itself to a good life again. Bottom line is get the family back together and work together as a team and make small but important gains in righting your world.
I understand the desire not to tick off the cousin....my sister was very reserved when I told her we were getting back together. But now two years later, it is like old times. If people REALLY care for you, they will over time get used to it.
Well, as I said it was going to be, this is getting long. Hopefully my message of restore the family first has been made. Everything else really will work itself out.