The win wasn't secured until Wallingham flashed on his last-rock takeout attempt in the 10th end, giving P.E.I. the steal and the win.
The result means P.E.I. will have a rematch for the final Pool B berth in the championship round with Northern Ontario on Wednesday. Smith fell 7-0 to Tanner Horgan earlier Tuesday to finish the preliminary round 2-4.
"We weren't very happy with the way we played this afternoon, and we had a bad third end there (against the Yukon), but other than that we played pretty solid," said Smith. "We always fight hard when we get down. We're kind of known for never giving up, and always coming after them.
"We got down three and we thought, 'Hey, it's no big deal.' We were down three to them (Yukon) a couple days ago and came back and won. Down three? We should be able to do it again."
The final Pool A berth will also be decided Wednesday, between Ontario's Mac Calwell and Nova Scotia's Matthew Manuel.
Calwell's Ontario team held off home-province hopeful Greg Smith of Newfoundland/Labrador 9-7 in the other tiebreaker on Tuesday, scoring a big three in the sixth end and stealing one more in the seventh to take control.
"We've been struggling — mostly it's been me having a hard time adjusting," said Calwell. "Just one of those off weeks. You can't be perfect all the time. But that game started to feel a little bit better. It's good to still have a chance here. We just have to win that second tiebreaker."
The top three teams in each pool, plus two teams with the next best win-loss records, advance to the championship round for a further four-game partial round robin from Wednesday through Friday, with their earlier records carried forward.
The six teams of each gender that didn't qualify for the championship round meet in a seeding round to determine ranking for the 2016 Canadian juniors in Stratford, Ont.
Quebec's Felix Asselin finished Pool A at 6-0 while Manitoba's Braden Calvert of Manitoba (5-1) and B.C.'s Paul Henderson (4-2) also advanced.
Alberta's Karsten Sturmay, Saskatchewan's Jacob Hersikorn and New Brunswick's Rene Comeau advanced with a share of top spot in Pool B at 5-1.
In women's play, B.C.'s Corryn Brown advanced with a perfect 6-0 record to lead Group B. Mary Fay of Nova Scotia (4-2), Saskatchewan's Kristen Streifel (4-2) and New Brunswick's Justine Comeau (3-3) also advanced.
Defending world champ Kelsey Rocque of Alberta (6-0) advanced from Pool A along with Ontario's Chelsea Brandwood (5-1), Quebec's Emilia Gagne (4-2) and Manitoba's Beth Peterson (3-3).