My problem is prior to her marriage Agnes doesn't seem to exist. I can't find a birth nor any census entries.
She could have been born and raised abroad if the family were with the soldier father - India comes to mind!
To further complicate matters there is another marriage for an Agnes Mary Bowgett in 1915 to a Thomas Paget. The marriage took place in the Maidenhead Berkshire registration district, which seems a bit of a coincidence.
If the two marriages are for the same Agnes it seems a bit strange she should marry for the second time using her maiden name if she was a widow.
The marriage certificate should show her father, which will give you a better idea if it's the same Agnes
Although, conventionally, she would use her previous married name, you can use any name you like as long as it is not for fraudulent purposes. Perhaps her 1st marriage was not a happy one?
There is a death of a Thomas Paget in WW1 on the GRO war deaths on FindMyPast but his name isn't on the CWGC website.
Unfortunately, not all names are on the CWGC site. There have been other postings on RC about people getting names added
Linda
PS The GRO
www.gro.gov.uk is the cheapest way to get certificates. Sites like Ancestry will charge nearly double

PPS Very few things are Impossible!!