So the authority that owns and manages Bent Cemetery, and maintains the records of who is buried there, cannot find a burial of Alexander Turner and Isabella Moffat. The logical conclusion is that they must be buried somewhere else.
Why do you think they were buried in Bent Cemetery? What is your evidence that Alexander Jr was born in 1909? Could this Alexander in fact be James Alexander Nevin, born in 1905? Did you ask the South Lanarkshire Council folk if they could find Alexander Nevin?
You really need to bite the bullet and get some certificates from SP. In particular
- birth of James Alexander Nevin or Turner in Glasgow in 1905. This will tell you whether or not you are looking at the right couple.
- death of Isabella Moffat or Turner in Govan in 1936. This will tell you the full names of both her parents, including her mother's maiden surname. It will also tell you whether or not she was widowed, which will help you to find the death certificate of Alexander, assuming of course that he died in Scotland.
Why do you think it was Bella Moffat's husband, whatever his name was, who became a master in the Merchant Navy? Have you looked for him in the Lloyds shipping registers? Who told you that he was Scottish?
AHA! I have just found a record of Alexander Turner in the British Merchant Seamen records on FindMyPast. Born 29 August 1880 in Garvagh, Ireland; next of kin, wife, Bella Turner, 70 Ann Street, Greenock. Not quite sure what his rating is - it looks like D keyman. A donkeyman operates a donkey engine, which is a small auxiliary engine. This looks much more plausible as an occupation for someone who was unable to sign his own name on his marriage certificate.
However there is another Alexander Turner, also D keyman, born in Edinburgh on 18 June 1880, who bears a passing resemblance to Bella's husband but has a different ID number.
I cannot find matching births of either of these Alexanders as either Turner or Niven in Scotland or Ireland, however. There is one in Edinburgh in 1880, but he was not born on either 18 June or 29 August.
There is a 20-year-old Alexander Nevin in Glasgow Dennistoun in the 1901 census. This is an exact match for both seamen born in 1880 (if they are indeed two different people). The original census can be viewed at SP and should tell you where this Alexander Nevin was born.
I've attached the photographs of the two merchant seamen. Do you think they are the same man?