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The bottom of the plug might be helpful to see also.
Are you seeing normal max RPM?
Perhaps your pressure gauge was just not cooperating on that cylinder?
If compression is still zero, I would suggest trying SeaFoam spray on that cylinder in case you have a stuck valve that might loosen up, but zero compression and running engine on something so new is a rare ocurrence.
If RPM and Speed are normal, no need to mess with it, the engine is fine.
If you were trying SeaFoam intake spray it actually goes in the intake while the engine is running. Instructions on the can.
You don't need it. If you are reaching max RPM and speed, you are not on a 0 psi cylinder .
No need to remove all the other plugs but It can affect your reading slightly.
Throttle position definitely affects your reading, and would show much lower without full wide open throttle .