I just unlocked this, so I can confirm a few things, but unfortunately, there is no easy solution.
The sort of good news is that you simply have to finish level 48 with 10 neighbors. The bad news is the game is REALLY hard so even with level select codes, for example starting at level 45, it will be basically impossible to beat level 48. You need to stock up on a lot of stuff for the last few stages but especially the final boss. (I strongly suggest you save all your pandora boxes for the final boss. And at least 1 or 2 of the red mutant positions. Also don't forget you can save scum, so with the "? mark" potions you can save right before you drink it, and keep reloading until you get the random effect you desire. I also found the secret flameflower from level 22 works fairly well on either spider bosses - and part of the final battle is a spider boss)
Another good news, you can lose neighbors. You have more than a few chances to get them back. First off, killing the big baby, there are 3 chances, will give you an extra victim if you are short one. Also, for every 40,000 points you will get a neighbor replaced (e.g., when your score hits 40,000, 80,000, 120,000, etc.). It's best to not lose them at all though. I was as low as 6 neighbors around level 30 but was able to gain 4 back by 42 and keep them all until the end of the game. Save scum of course. Do it when you feel safe, you just have to use your instincts. Also when a victim seemed impossible to save based on my save location, like running down as fast as I could, I instead just went left, and the baddies de-spawned. So try various things like that if it seems like that if you get in a tight spot and have trouble getting a victim.
When they turn into werewolves you do indeed lose a victim. They are on a timer and will always turn into one. I recommend looking up one of the super really detailed guides for SNES or Genesis on GameFAQs for the werewolf levels. The GameFAQs guide I was using tells you exactly how many seconds you have to reach the neighbor and which neighbor it is. Sometimes it's as little as 7 seconds!!! The very last level, 48, it is key to have all 10 (9 before the boss, and 1 after), and guess what? You have TWO victims that turn in werewolves in less than 37 seconds. You pick up a pair of blue shoes in level 47. Before you step into the portal for level 48 have them selected and ready to use!!! You will need to use either a video or a detailed description of how to reach the neighbors, it's is convoluted. With the blue shoes on, and the route memorized you will have mere seconds to spare. So save the moment level 48 spawns. You can then try over and over again till you get it. It took me about 9 tries before I was able to save them in time. The entire run could come crashing down right here, so in many ways, these two neighbors are the most vital
This is not my video, but this is the one I used for level 48:https://www.youtube.com/d014527a-926e-4d3f-9cfd-70795f644fdb
The bonus levels usually give you tons of extra supplies, far more than average levels, and they tend to be easy. I highly suggest doing them all
even though the bonus level achievement is glitched.
(edit: its fixed)