Windows Defender isn't very good, but that shouldn't be relevant since it shouldn't be running at all when security center detects a third party antivirus. Can you just verify that when you're restarting, you're pressing the "restart" button in the start menu, not shutting down and then manually turning on the computer? Shutting down in Windows 10 actually saves the kernel state to disk, meaning you are not actually restarting.
I would also try running Zemana Antimalware (free second-opinion scanner, https://www.zemana.com/en-US/AntiMalware) to help rule out the possibility that the "Antimalware Service Executable" you are seeing is actually a hiding malicious program.
If none of that works, there's some other things you can do like looking for suspicious startup items in Autoruns (part of Sysinternals), booting in safe mode, and using System Restore. However, it might be easier to do a clean install of Windows if you're not able to resolve this issue.