But McGregor now has three wins from five starts since taking over from sacked coach Steve Price, and as the Dragons edge to within one win of the top eight, he is strengthening his case for a permanent contract.
Billy Slater took another step towards Ken Irvine's coveted all-time try-scoring record of 212, by leapfrogging Andrew Ettingshausen into outright third on the list with his 166th career try.
Irvine's mark long appeared to be one rugby league record that may never be broken, but if Slater can play for another three seasons and maintain his current strike-rate, he remains a chance to make history.
Slater's first half try got the Storm within four, after Marshall and Widdop inspired the Dragons to their fastest start of 2014.
A crisp Marshall long ball put Dugan over after six minutes - showing Melbourne what they missed out on - with the Storm one of the suitors who missed out on his signature mid-season.
Then it was Widdop's turn to rub it in, chipping and regathering with a lucky bounce to score against his former teammates.
A major turning point in the match came shortly before halftime when referee Ashley Klein penalised Storm prop Bryan Norrie for an obstruction, even though he was immediately tackled with no try scored.
Storm captain Cameron Smith argued at length that the call was inconsistent with how the rule has been interpreted all season, but it didn't help Melbourne who find themselves in unfamiliar territory at this stage of the season in ninth place.
Dugan hyperextended his ankle before the break but battled on, producing a key play to beat his man on the outside before flicking a miracle flick pass for Jason Nightingale to cross.
Dugan appears as though he will be fine for NSW selection, while fellow Origin candidates Trent Merrin (Blues) and Will Chambers (Queensland) returned from injury to turn in impressive performances.
Storm coach Craig Bellamy said his team "put two points on a silver platter" for St George Illawarra with a lacklustre start to the second half.
However he branded the obstruction call against Norrie before halftime as "ridiculous."
"I'm highly confused. How many times have you seen that in a game? I think Cameron made the point, I don't think he obstructed anyone because he didn't score," he said.
"Next time that happens I want to see a penalty ... but it won't. It'll probably be a one off."
St George Illawarra skipper Ben Creagh said McGregor's coaching style has brought the best out in his team.
"What Mary expects from us at training and what he expects from the game, he's pretty clear on that and the guys have bought into it," Creagh said.
- AAP
