I'm sorry, my heart cries for you after loosing so many of your little ones.
I like the idea of getting the necropsy too, not just to pinpoint what have happened. It may be nothing more than extremely genetically unhealthy mill budgies, it may be something you don't realize that budgies are more susceptible to than the rest of your flock. If I were you it would give me a lot of closure to have a real answer as to what may be going on, and, if the issue was just poor breeding practices for the budgies sold in pet stores it might help you move past your doubts.
Edit:
And if your parents are against the idea try to convince them that something could be going on that could endanger your other birds. I'm not trying to scare you, that may or may not be the case, but if I remember correctly in the past your parents have thought your budgies weren't "worth it" (or am I thinking of someone else? Sorry if I am.)