the strength of the Falklands defence forces would become irrelevant if the Argies occupied Ascension first.
Airborne surprise assault using three or four 747's arriving unannounced, followed by a few Super Etendards tankered in once the runway was secured.
No need for air defence - they'd just need the anti-ship Etendards
They wouldn't need to hold it for long - just long enough to break the supply line while they invaded the Falklands and dug in. If they did it just before the South Atlantic winter began, they'd have several months to fly defences into Mount Pleasant
The USA may moan, but if it was presented to the UN as a temporary measure to protect themselves while the "reposession" took place they could spin it out long enough until they felt it was safe to withdraw