There is a reference to US assistance in Ken Connor's book, "Ghost Force". He writes that he was in the US at the time and was part of the arrangements to get US Stinger air to ground missiles and better radios to the SAS.
In relation to the US mercenaries there is a reference in Martin Pegler's book, "Sniper: A History Of The US Marksman" to two snipers captured at Goose Green that had caused casualties. Pegler's source was a personal interview.
...they had proper commercial scoped rifles and they had caused some heavy losses but neither spoke Spanish - they spoke English with American accents but didn't say much. They knew we were pissed off with them. Some of the lads took them away and we never saw them again. We all knew they were killed but no one spoke about it.
I think I am correct is saying that it has now being revealed that a lot of US intel was passed to the British including satellite photos as I think the US repositioned one for the purpose. However, I cannot recall where I might of read that so I could also be wrong.