Long time lurker, appreciate all the knowledge and contributions on Flyertalk.

I have read this thread with interest and would appreciate if anyone could point me to a specific post that offers some official confirmation (or information I could provide to an agent if in need) for the following scenario:

JFK-LHR-TXL on BA and totally separate ticket TXL-MIL on AB

If I understand correctly, even though flights are on separate tickets if delayed and connection missed because these are both OneWorld airlines they would still get me to my final destination, Milan?

Additionally, this thread references getting agents to check through all boarding passes. I have not had luck with that in the past. For example, would I ask BA at JFK for my TXL-MIL boarding passes?

I appreciate your help!