Hey everyone, I'm overwhelmed with the help so thank you. Your ideas give me a lot to research and think about that I hadn't before

Also, what records/when Charles used each version of his name?
In his marriage certificate and all electoral rolls he used Charles Joseph Sargent so from 1924-1953. I only discovered the second name when I ordered his full death certificate. This seems to be the only mention of the name, I have a feeling it could be his birth name and he changed it to Charles. There was a William Joseph Sargent born in South Dublin in 1886 but it's only a text record (Irish Birth Index) and I'm unsure of how to proceed in finding more information about this.
Going back to the William Augustus Sargent of the Army records having been discharged in 1869 and saying he would be residing in Stepney, London, have you researched this birth of a William Augustus Sargent in Mile End Old Town (which I believe was part of Stepney), London in 1893 in case of any links back to the 'army' William?:
https://familysearch.org/ark:/61903/1:1:2F9K-NZB
I haven't seen this before but thank you. I will definitely get researching this.