Cosker also trapped Ben Scott for 18 as Middlesex slumped from 77-3 to 129-7.
Tom Westley fell first, beaten by some late movement to offer wicket-keeper Ben Scott his first of five victims, before Grant Flower edged to slip.
Malan was caught behind driving for 47 off 81 balls and Ben Scott went the same way for a duck when he followed a wide one in the Jamaican's next over.
The game looked to be heading for a draw after Malan and Ben Scott (44) added 99 for the sixth wicket, until, soon after the lunch break, acting Middlesex skipper Eoin Morgan struck a deal with Essex counterpart Mark Pettini.
Walker's injury, after he went down awkwardly in a tackle with Senior, was a cruel blow with Shaun Briscoe, Ben Galea, Scott Murrell, Ben Cockayne and Jake Webster already absent for the Robins.
Edinburgh's Geoff Cross, Roddy Grant, Ben Cairns, Scott MacLeod and Greig Laidlaw will also have been particularly disappointed to have missed out on Andy Robinson's main squad along with Glasgow's Rob Dewey.
Sir Ian McGeechan also has options in Stuart Hooper, Ben Skirving and possibly academy graduate Scott Hobson, who has been out all season with injury.
But the visiting team dominated large periods of the match and continually tried to force their way back into the contest, with Damian Scannell a particular menace from wide areas, while Ben Williams denied Francis Laurent and Scott Spencer.
But in a Scrum V Radio interview, the Wales coach also named twice-capped Wasp Ben Broster and Cardiff Blues' uncapped Scott Andrews.
Scott said Miller and UCLA coach Ben Howland were told in a pregame chat on the court, which takes place before almost every game at every level, that inappropriate conduct on the sideline would not be tolerated.
In short, the message should be particularly clear to Senators Olympia Snowe, Roger Wicker, Bob Corker, Scott Brown, Jim Webb and Ben Nelson who are up for reelection in 2012: Those who believe the United States must practice the authentic Reagan philosophy of "peace through strength" will accept no substitutes.
So, too is the host, Scott McGrew, and guest interviewee, Ben Parr of Mashable.
CNN's Ben Brumfield, Phil Gast, Dan Moriarty, Scott Thompson and Janet DiGiacomo contributed to this report.
Ben Farrar's try put the Dragons ahead, Scott Dureau kicking the conversion.
The Steelers defence is now complete with Kevin Reiter and Dan Green sharing goaltending duties, and Bolibruck, Scott Basiuk, Randy Dagenais, Mark Thomas and Ben Morgan patrolling the blue line.
Scott also revealed the thinking behind the recent signings of Dagenham and Redbridge trio Ben Strevens, Sam Saunders and Danny Foster.
Scott Murrell put Rovers back in front, with Dobson's drop-goal and Ben Fisher's late score sealing the win.
By the time he was 19, Holland was playing with the house band at Ronnie Scott's, a legendary jazz club in London, where he got to play with Ben Webster and Coleman Hawkins.
CNN's Moni Basu, Brian Todd, Ted Rowlands, Carol Cratty, Mike Mount, Ed Payne, Scott Bronstein, Tom Cohen, Shawn Nottingham, Susan Candiotti, Deborah Feyerick, Phil Gast and Ben Brumfield contributed to this report.
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