Hi Amanda, I haven't been able to find a cemetery listing for William Mason Mallinson in Chichester unfortunately, so nothing totally solid there. You mentioned you have his death certificate. Whereabouts exactly did he die in Chichester? It seems far from where he was living in Paddington, London in 1911 with Mary.
Have you been able to track down a marriage for him to Mary at all. It seemed that he never really divorced Alice and they just became estranged. It just comes to an abrupt dead-end when she goes to Canada. I'd really love to know if she stayed (though as mentioned before, there are no burials found yet for her despite our efforts to find them and Alberta, Canada's privacy laws are very restrictive when trying to get records for BMD's.) I'm not sure either if she came back to England, given the brutal winters we have here in Alberta!!
As for Cecil William Mallinson, there is a reference for what could very possibly be him in 1926 living at 5 St. Leonard's Road, Ealing, Middlesex (looking at 1926 Kelly's Directory). In 1916, he was with the Bedfordshire Regiment according to the London Gazette. He married Evelyn Lucy Crawley December 21, 1907 in Ealing and they had two children: Bernard Cecil Mallinson b.1909 and Cedric Howard Mallinson b. 1912 (who emigrated to NZ and later lived in Aus.) I don't have a death for Cecil, nor his wife Evelyn, yet, so I don't know what happened to them post-1926.
Kind regards,
Jilly