"After he died, the family discovered paperwork that showed his name to be William Titley, and that he had abandoned a wife & children in Victoria before coming to WA"
"I still don't know why he left Victoria and took on the assumed identity of William Bell in WA."
Haven't you answered your own question there?
The William TETLEY that you have on your Ancestry tree is not your man. He lived with his wife Mary Kate until his death in 1923 and his Will is digitised on PROV, he was born c1852.
What is the evidence you have that William BELL/TITLEY abandoned a family in Victoria?