I can't find out anything further for you unfortunately unless I pay to view the 1911 census entries. As you know, SP is pay to view only and the only source of 1911 census info. I have used some of my credits to view Ellen's death to help you along

I am not sure about the entry for William in 1911. There is a William in 1901 of that age who could be the same one as the one in 1911:
Philip Allan 30
Mary Jane Allan 26
Thomas Allan 25 Brother
William Allan 20 Brother Serving Machine Maker - Machinist b. Ireland
Philip Allan 2
Mary Jane Allan 1
Philip Wilson 30 Visitor
David Smith 21 Visitor
Address: 43 New Street , Old Kilpatrick
Without having found William in 1901 in Ireland or Scotland, possibility he went further afield or may have died young?
John Allen does look a possible but his entry would need to be checked.
Monica