I would suspect that with such a high failure rate on separate devices that the parts you are using are defective. I would try a screen that you know is from a separate supplier if available and see if the problem persists.
Hello Pedro, the alcohol will not damage anything. Just apply it carefully in the areas where you see corrosion, you want to focus on the battery and lcd connectors in particular and may be a good idea to clean them either way as sometimes corrosion can not be seen without a microscope. A good place to source a new screen if required is iFxit(I am not sure if they have stock). If not you can google and will find many suppliers, I would stick with only high quality parts and do separate reviews on whoever you choose.