You could possible use a mod that make NPC protected if you are worried about NPC's getting killed. I believe the only way they can die is if you kill them unless that NPC has the essential flag on it.
There is no mod that is 100% reliable, The game is not even 100% reliable. Likely the closest mod that is reliable is the unofficial patch from Arthmoor
Most mods are compatible with current game saves but if you want to remove the mod it is recommended to go back to a game save before that mod or mods were added.
For the most part using scripts will not cause issues since mod makers have become quite good with keeping the uses to a minimum and avoiding the scripts which are heavy on the performance.
If on the PC game save bloating is not much of an issue as it is on the consoles and I believe the Unofficial Patch might help prevent some of it.
All the posts that I have read are trying to help you. No need to be rude to them, Next time you might not get any help at all.
It might help us to know what mods you have if you are worried about mod compatibility. The more mods you use the more likely you will have compatibility issues. Most are likely minor and can quite often be resolved by using a bash patch.
As for your request of the mod I have not heard of a mod that does that the closest was the mods from Arthmoor that were suggested earlier and he is one of the top mod makers and his mods are quite reliable. Serethil did have suggestion for a mod but I have never heard of it or used it.
Maybe someone will make that mod for you but it might be up to you to make it.