The court heard that Howson's relationship with Shakira Ramdihal turned sour after she became pregnant.
Howson starred as Heathcliff in a film adaptation of Wuthering Heights released last year.
Luciano Becchio's eight-yard strike, and neat finishes from Gradel and Howson propelled Leeds to victory.
Howson is best known for paintings of the Bosnian war and the singer Madonna.
The pair struck up a friendship after Bowie purchased two of Howson's paintings depicting the Bosnian war.
BBC: Peter Howson's wife to sell rare Bowie and Madonna work
But Leeds were rewarded for their determination when Howson scored to secure a fifth away league win.
Franck Moussa's close-range finish gave Rovers the lead but a minute later Howson's 22-yard shot levelled matters.
Designed by Peter Howson, the stamp depicting steam power is due to go on sale on January 12.
It was left to Howson to put the icing on the cake, running clear to finish with aplomb.
There are also 10 sketches of Madonna and early drawings of Howson's teachers.
BBC: Peter Howson's wife to sell rare Bowie and Madonna work
Howson's barrister said his client had been Mr Quigley's best man and did not want to offer any mitigation.
BBC: Driver jailed for crash death of soldier Gordon Quigley
The points were secured when Luciano Becchio played Howson through on goal and he coolly completed a maiden career treble.
Howson had hauled Leeds level 2-2 on aggregate with the opener when he drove in a shot after 10 minutes.
Extra-time loomed before Howson angled in an 18-yard strike with his left foot to send Leeds to Wembley for the final.
The court heard that Howson had agreed to drive a friend's car home after a day out watching football and drinking.
BBC: Driver jailed for crash death of soldier Gordon Quigley
James Howson, 24, of Haslewood Close, Leeds, earlier admitted racially aggravated harassment and was due to be sentenced at Leeds Magistrates' Court.
Pop icon Bowie posed for Howson at his London studio in 1994.
BBC: Peter Howson's wife to sell rare Bowie and Madonna work
The 10 Bowie drawings are from 1994 when he posed for Howson.
BBC: Peter Howson's wife to sell rare Bowie and Madonna work
Howson became established in the 1980s as one of the "New Glasgow Boys", a clique of painters concerned with portraying social issues.
Driver Christopher Howson, who had no licence and was twice the legal alcohol limit at the time, admitted causing death by dangerous driving.
BBC: Driver jailed for crash death of soldier Gordon Quigley
After Howson's rapid reply, the Leeds fans grew increasingly frustrated until Becchio put them ahead again with a close-range finish following a Leigh Bromby knock-down.
Jermaine Beckford flicked on a header to Howson, who spread ball to Freedman on the left flank before continuing his run into the Carlisle area.
The six paintings feature in a Glasgow exhibition by Peter Howson.
During the Balkans conflict, Howson became the official British war artist.
Mr Howson has returned to the Balkans, sponsored by the Times.
The visitors' pressure eventually told when Connolly's cross made its way through the area to the back post, where Howson was waiting to convert his third of the season.
Howson, 50, who studied at Glasgow School of Art, is renowned for his powerful figurative work which has been collected by celebrities such as Madonna, David Bowie and Bob Geldof.
Forest remained on top for the remaining 10 minutes of the opening half but found themselves behind five minutes after the restart when Robert Snodgrass and Eric Lichaj combined to play in Howson.
"We could be talking about a team not making the playoffs and missing out on millions of dollars in playoff gates, " Howson, the Blue Jackets' general manager, wrote on his blog on Feb. 2.
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The tension increased the longer the game went on and, just when it seemed extra-time was inevitable, Howson shot into the corner of the net to put Leeds one step away from promotion to the Championship.
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