A reader of this group has sent me the following very interesting information:
1. He located "the marriage of Edward Lascelles and Ann Elizabeth Rosser at St James's church, Bristol on 3 November 1818 - the marriage was by licence, and both parties signed the register - he was described as "Edward Lascelles of Westbury upon Trim, Esquire, bachelor"; she was a spinster of Bristol. The witnesses were Ann and William Lodge."
2. In "...his probate record in the Prerogative Court of Canterbury from 1844 ... he is styled His Highborn Count Edward Von Lascelles, but indexed as The Honourable Edward Lascelles commonly called Viscount Lascelles -- this records a deed between "the Count" and his wife, born Baroness Phillippine von Lutgendorff, dated at Wurzburg, 2 May 1838. This was essentially a post-marital contract, registered with the Bavarian courts and then used to obtain English probate."
3. A 2010 book, "A Swindler's Progress: Nobles and Convicts in the Age of Liberty" by Kirsten McKenzie, contains much detail about the Viscount and his chequered marital history. The first marriage (Ann Elizabeth Rosser, 1818) was to a barge-owner cum publican's daughter and was certainly not welcomed by the Lascelles family. It seems Edward was sent off to the Continent and lived there in quiet retirement - he was given a place at the embassy in Vienna, where he met Phillipine Testa (sic) "a twice widowed Bavarian baroness" who became at first his mistress. After Ann Elizabeth died at the age of 38, they were free to marry (her second husband had died in 1814). The author suggests that "Louisa Rowley" never existed, and that the 1821 marriage was invented by the Lascelles family as a cover for the real story."
So it seems that the marriage with Louisa Rowley, even if she existed and it ever occurred, was probably illegal. I wonder whether it was because of his checkered career that Gentleman's Magazine in 1841 didn't even know that he was dead.