E-outstanding 2 Pcs Recessed Door Window Magnetic Switch Wired Home Security Alarm Contact Sensor, White

I'm so sorry to hear that. If you had wired sensors the installer should have reused them. They have modules that would have replaced the Panel "A" and send the signals wireless back to your new alarm. PROSIXC2W or 5800C2W are likely what are compatible with your system. The door/window sensors might be recessed into the doors such as: https://www.amazon.com/outstanding-Recessed-Magnetic-Security-Contact/dp/B08XW4VJJ8 To answer your question the most obvious thing that can be removed is "C" -- you can replace it with an ethernet switch if you want wired networking in your home. D, B, E can also be removed without harm.