Hello,

Sorry for your troubles in getting a simple question answered.

You should be getting the caviar service on your flight, I would guess close to 99.9 percent chance. I cannot imagine why they would not have it.
I have never flown Cathay in F from the states and NOT seen the trolly out for this service. They have always done the trolley and it is very nice. This is in probably 20 F flights from the US.
It is part of the first meal service (lunch, dinner). They really don't have a second service until the end of the flight (often breakfast). There is a snacks menu for use between the first and second meals. If you really want something from the first (or second) meal in the middle of the flight, they will usually do it for you too.
Just ask. They are very obliging.

You CAN get a second caviar serving later in the flight. My wife always asks for it and gets it for breakfast (!), along with her eggs! They serve it on its own plate, with crackers and the same little sides. You can also get the salmon with your eggs for breakfast.
I really think it depends on whether or not they have enough "left over" from the trolley service.

Enjoy!