Nacewa, having switched to fly-half, launched a high kick which Blues Mustoe failed to hold.
That hope looked to have evaporated when Mustoe scorched through within two minutes of the restart.
Cardiff sacrificed Leigh Halfpenny for Richard Mustoe, before Edinburgh lost Kyle Traynor, with Gavin Kerr coming on.
Myler nudged the Saints back ahead with a difficult penalty after Mustoe was isolated at a ruck.
Michael Paterson also crossed and Richard Mustoe went over for the bonus point with the game's last move.
Chris Czekaj, Tom James, Richard Mustoe, Aled Brew and Rhys Williams provide competition for the two wing berths.
The same player put in a try-saving tackle on Richard Mustoe as Blues began the second half with intent.
Richard Mustoe got the Dragons' crucial fourth try but the Italians hit back.
Ospreys wing Richard Mustoe has joined on a season-long loan, a deal that worked well with Aled Brew last year.
Bradley Davies, Richard Mustoe, Richie Rees, Tom Shanklin and Tom James all claimed tries, with full-back Ben Blair kicking another 14 points.
Full-back Jerome Perical failed to collect Richard Mustoe's speculative kick up field, and Wyatt pounced to hand the Dragons a 5-3 lead.
Richard Mustoe was halted at the line, but Rhys Thomas was on hand to finish off and collect a bonus point of their own.
Blair's late long-range penalty brought the hosts level after Nacewa's try, scored after Blues wing debutant Richard Mustoe failed to hold on to a high ball.
Laulala's long pass set up an overlap on the right and a couple of well-timed deliveries from Davies and Mustoe sent Xavier Rush charging into the 22.
Leicester, with South African Derrick Hougaard at fly-half instead of Flood, won 19-15 against Bath, while the Blues beat Sale thanks to a last-minute try from Richard Mustoe.
Sweeney missed the chance to steady the ship when his penalty drifted wide and Treviso capitalised when Sgarbi beat Mustoe with ease to score the Italians' third try.
Even then the drama was not over as Maddock, possibly still slightly dazed, took the conversion from in front of the posts but saw his kick charged down by the on-rushing Richard Mustoe.
It was his centre partner Casey Laulala who lit the Blues' touch paper, though, splitting the Saints midfield defence on halfway, the attack carried forward for Mustoe to pile over from close range.
But the reprieve was only fleeting, as soon after the Dragons backs were left looking at each other when a high kick was allowed to bounce and Blues wing Mustoe gratefully gathered to go under the posts.
Most of those Welsh tries came in the win over Chinese Taipei, with Scarlets duo Alec Jenkins and Jon Edwards crossing twice, and Tal Selley, Rhodri McAtee, Dai Flannagan, Richard Mustoe and Lee Beach also touching down.
Veteran flanker Colin Charvis scooped up loose possession, but just as the threat seemed to have fizzled out Blues wing Richard Mustoe flicked it back along the floor and Fussell raced through to gather and speed away.
Glasgow, having used all their replacements, suffered a further set-back when Forrester was carried off on a stretcher, followed by limping skipper Alastair Kellock, and the 13 men left were unable to prevent the Blues claiming a bonus point as Mustoe galloped over from 50 metres.
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