I think you may have more information to work from than you think.
Write down everything that you know about each individual you are trying to find, no matter how small or insignificant you think the information is. What you have been telling us is very confusing, don't know if your father is alive or not. You do not need his permission to get certificates on your grandfather.
For your grandfather, you can access his military records at
www.collectionscanada.cago to library and archives Canada, Soldiers of the First World War, under search - enter name, under results - access Joseph E. Seeley, under Item Display - How to consult a file on site or order a copy of the file. You can order this file through a secure site on line. If you have problems accessing this through collections Canada, google Soldiers of the first world war.
The description on the file describes the same features that you stated above so order the file and see what is in it.
You know that he married Kathleen McDonnell but where, was it in Liverpool, England. If so, you should be able to find the marriage certificate and order it. You also know that he died in 1972 and there is a headstone. Where is he buried. That should lead you to the avenue to order his death certificate. Most countries allow family members to order the certificates.
You have the names of your father's siblings. Follow up on those. Do you know where they lived and also where they died. If you have this information, you may be able to access information.