What you could try to do is saving your newly moved in reaper as a family of it's own to a file in youre Electronic Arts folder, making a copy of it, putting your old savegame back, and than add that new deadly family to your home. Maybe for the save you can change the name of the reaper for in case the game will have problems with double names (normally is haven't of course, but with reapers you can't be save enough) and put that reaper back in your previous game.
Now, you can test if the reaper from your 'alternative future' will bug with the NPG reaper from your previous savefile or not. If it works, youre sims can die while the old reaper picks them up to collect their souls or something, but you also have a playable reaper for your own.
I'm really not sure if this will work or not, but after reading your post, this was the first thing that came up in my mind. So try it, if you like and please respond to me if it worked for you. I really like to know.
Alright, it was formerly known that you couldn't get children with the Grim Reaper after patch 1.3. However, I advocated a long time that it was without problems possible to make him move in with you through friendly interactions, after which he would be counted as a normal sim and you could get children with him, along with the completely black skin and all.
I tried this again recently for the first time after the latest patch and it's still possible to make him into a normal sim and get kids with him, but where before there were 3 Reapers acting at a given time (according to my personal testing) and a new spawning after one of them moved in, it's apparently now only one. And this guy don't regenerate after he's moved in, effectively making it possible to kill death completely (if he dies after moving in with you). Of course I tried that a great plenty and noone can die afterwards. Or more accurately, they die, but their corpses lay down in the street without anyone picking them up or giving them tombstones.
This goes the same if he just moves in with you without dying since he's then counted as a normal sim and won't go out reaping sims. Now, the reason for this topic is to say to everyone to be careful when experimenting with this from now on. I lost a good 2h when I had to recover my old save game, which felt horrible. Also, can somebody verify whether this is a game-wrecking glitch just in my game, or if it's something that EA has put in to make it harder to get reaper babies?