Will this work if my glow plug has already snapped off flush in the head?

This tool is designed for extraction of intact or partially stuck plugs. If the plug has fractured below the surface, you'll need additional tooling like a screw extractor or EZ-Out kit to remove the broken piece first.

What do I need to do before using this on a seized plug?

Apply penetrating oil and let it soak for 24 hours minimum. Heat the head with a heat gun if possible — thermal expansion helps break corrosion. Then use steady, deliberate pressure rather than jerking force.

Does this tool come with a socket or breaker bar attachment?

The tool itself is ready to use — you'll supply your own 3/8" drive ratchet or breaker bar to apply torque. This lets you control load exactly how you need it.