Cousineau was 20 for 30 passing for 219 yards for Montreal (7-1), which trailed 7-0 before getting its first points of the game via a field goal in the third quarter.
Laval (7-1) started the game with four straight rouges before Boris Bede hit a 37-yard field goal at 11:11 of the third quarter to pull the Rouge et Or ahead 7-0. Laval added two points on a safety five minutes later.
The Carabins held Laval quarterback Hugo Richard to 138 yards and an interception.
In Winnipeg, meanwhile, Manitoba quarterback Theo Deezar threw three touchdown passes to Nic Demski as the Bisons clinched a playoff spot in the Canada West division with a 50-31 victory over the No. 2 Calgary Dinos.
Deezar finished the day with 210 yards and an interception to go along with his three TD passes, Demski racked up 126 yards on four catches and threw one complete pass for 43 yards, and running back Kienan Lafrance rushed for 57 yards and a touchdown on 19 carries for Manitoba (4-4).
Andrew Buckley and Jimmy Underdahl ran in touchdowns for Calgary (6-2). Receiver Rashaun Simonise also scored.
X-MEN 18 AXEMEN 17
ANTIGONISH, N.S. — Jeremy Ford kicked a 36-yard field goal with seven seconds left to give St. FX the win over Acadia in the AUS semifinal on Saturday.
MUSTANGS 25 GOLDEN HAWKS 10
LONDON, Ont. — Yannick Harou scored Western's only touchdown with 2:03 left in the fourth quarter to give the Mustangs a come-from-behind win over Laurier in the OUA quarter-final.
GEE-GEES 46 LANCERS 29
WINDSOR, Ont. — Derek Wendel threw for 264 yards and two touchdowns and running back Bryce Vieira added two more scores on the ground as Ottawa routed Windsor in the OUA quarter-final.
STINGERS 30 REDMEN 28
MONTREAL — Olivier Dupont rushed for 113 yards and a touchdown as Concordia (5-3) edged McGill (0-8).
VERT ET OR 35 GAITERS 14
LENNOXVILLE, Que. — Quarterback Jeremi Roch completed 29-of-40 passes for 397 yards and two touchdowns to lead Sherbrooke (5-3) over Bishop's (1-7).
RAMS 35 GOLDEN BEARS 31
REGINA — Atlee Simon ran in an 11-yard touchdown with 2:40 left in the game to give Regina a come-from-behind victory over Alberta (3-5). The Rams (3-5) finish fourth in the Canada West standings and will meet Calgary in the semifinals next weekend.
HUSKIES 38 THUNDERBIRDS 18
VANCOUVER — Drew Burko threw for 291 yards and three touchdowns and backup Kyle Siemens added another TD toss to lift No. 8 Saskatchewan (6-2) over UBC (3-5).