Theo, I think Joachim has already given a significant hints to you. Whatever, currently you are in a position where the edit form did show the contents for that particular field, but the contents of that field are not shown anymore on the main view.
I just want to make sure that every backend settings are correct and there is no such hack in your core module. Can you please share me your source so that I can make a local setup here and can look into this together? Do not think you will have to pay me to check the problem. I just want to make sure what is there and how we I can start debugging there.
Some general information which you may check but I am sure that these are not the case:
As the notice is showing an Undefined property notice, so first you have to check if that property exists in the module file, before you do any operation, you should debug your code with the particular functions, notice describes that $id is not defined within the file, or it is not getting the $id value. So after changing it into lower case it will not effect of editing records, nor add new ones.
In other case you have mentioned that when you click the Edit link in the view that comes with this module, the edit form opens but its fields are empty, for this you need to add fields properly, or you can join tables using custom query within the view.
I will wait for your message to check further, please let me know your time, I will manage my time.