For the FAA ATPL written test, no sign-off required (as stated above) no proficiency required. It is a written test like all the other written tests and it is valid for 24 months.
So unless you already have 1500 hrs or expect to reach it soon, there is no reason to take it.
There is no such thing as ICUS under FAA, you are either PIC or you are not.
At least the term does not exist in the uS.
You can log PIC under the following;
*as sole manipulator of the controls (rated and current in category and type)
This could imply that you could be under supervision of a more experienced pilot.
*when receiving additional instruction in an aircraft you are rated for eg
instrument instruction, commercial instruction, recurrency, check-outs etc etc.
* as flightinstructor on a training flight.
* as assigned PIC for the flight