As you say, if he was freelance they didnt need any excuse therefore his version of events doesnt stand either.
Bottom line is we will never know the real story
His version of events is plausible to me because 1) my parents have read The Express for years, I know the paper's attitude well, and this fits with it
2)the pictures referred to exist, and I have seen them
3) Shenton's been lamenting the way newspapers have been cutting their arts coverage and getting rid of critics. He has worked for The Express for 11 years. If they'd decided to let him go for another reason, that would have fit quite nicely into his narrative about the state of arts criticism. He doesn't need to make up anything dramatic.
4) Truth is stranger than fiction - you couldn't make this up!
There are two sides to every story, that is true, but given the way The Express has justified extremely unethical decisions in the past - just look at their rampant scaremongering, and the way they libelled the McCanns, not to mention the Diana conspiracy theories they peddle - there's no particular reason to think their side of the story would reflect well on them.