Swans boss Kenny Jackett gave 17-year-old local product Shaun MacDonald his first League start on the right of midfielder in a bold move after Leon Britton withdrew with an abdominal injury.
Lee Trundle was also restored to the front line after being dropped to the bench at Oldham last Saturday, while Fallon kept his place alongside him.
Garry Monk also returned after shaking off a back injury to partner Kevin Austin in the heart of the defence, while Alan Tate and Sam Ricketts operated the full-back berths.
Kevin McLeod, who came close to joining Southend in January, kept his place on the left of midfield with Andy Robinson and Kristian O'Leary in the middle.
There was again no place for skipper Roberto Martinez on the bench.
Southend fielded the same team for the third match in a row as they went in search for that elusive win to confirm promotion after four games without a victory.
The match started at a frantic pace, but it was the visitors who edged the opeing 15 minutes as Kevin Maher's corner found Adam Barrett at the far post, but the defender's goalbound header was parried out by Willy Gueret.
Then Lee Bradbury, Mark Gower and Luke Guttridge combined to set up Eastwood, but his header was cleared off the line by tate.
Trundle eventually stemmed the tide as his first shot flew over before his second effort was blocked on its way to goal.
As the game continued at a frantic pace with both sides going all out for victory, Robinson's surging run ended with a fierce shot which Darryl Flahavan had to tip over.
Swansea had turned the game in their favour at this point as they took the lead in the 19^th minute when Fallon picked up a loose ball on the edge of the area to fire a fantastic finish into the top corner for his first League goal at the Liberty Stadium.
Fallon was booked for removing his shirt, but it was a small price to pay.
But the lead lasted just eight minutes as Gower cut in from the left to feed Eastwood who waltzed past Monk before slotting the ball beyond Gueret.
In a thrilling first half, Swansea were back ahead in the 34^th minute as Tate picked out Fallon with a perfect cross from the left and the New Zealand striker despatched a great header into the same top corner for his second of the afternoon.

Half-time: Swans 2 Southend 1

Trundle nearly increased Swansea's lead four minutes after the break when he forced a great save from Flahavan. From the corner, Fallon's header down found Trundle again, but his goal-bound shot was blocked by Maher with the Swans well on top.
Flahavan was called into action again in the 55^th minute when he was forced to save from a quick trundle free-kick after a foul on the impressive MacDonald, while Fallon had another header saved.
At the other end, Barrett tested Gueret with a low drive which the Frenchman beat out.
But with both sides finding it hard to maintain the amazing pace of the first half, it was Southend who snatched the equaliser in the 63^rd minute.
Again it was Eastwood who did the damage when he turned Austin inside out before slotting home his 23^rd League goal of the season beyond Gueret.
Jackett sent on Leon Knight for Trundle in the 71^st minute before McLeod was stretchered off with what looked like an ankle injury with eight minutes to go; Tom Williams replacing him.
There was no more action, however, with neither side able to make the breakthrough. By then, however, news had already filtered through that Brentford were drawing at Hartlepool to guarantee Southend an automatic place for their second successive promotion.
As Southend were left celebrating with their 2,000 fan on the final whistle, Swansea were already turning their attention to next Saturday's make-or-break clash at Chesterfield.
The Swansea players did return to the pitch for the traditional lap of honour on the final home game of the season, but the focus remained fully on Chesterfield.