Unless USB keylogger is present which will log the disk encryption password when user is unaware of it...
I've already seen lockable boxes around computer ports on some older model workstations. Combine that with some tamper proofing on the keyboard, and you've probably bought yourself a little more security against any intruder who has limited time with physical access.
Not if you can get access twice. A variety of "evil maid" style attacks exist. For example, imagine I replace your FDE's bootloader with a version that appears identical, except it also logs your passphrase. Then I come back, read the passphrase, decrypt, and win.
For scarier thoughts, imagine I know how to control the Intel Management Engine, and attack that instead. That's not covered by FDE.