Sue
My family stayed on the Isle of Man and moved from our spiritual home of Marown to Lezayre and then to Douglas so not too difficult to follow.

My first David married twice Ann Cottier (1780) then Alice Quine in 1802
Ann’s first son David died of smallpox aged 16 years and is buried in old Marown cemetery with following Monument inscription:
In memory of David Cretney, who died on the 29th of Sept 1795, in the 16th year of his age. Son of David and Ann Cretney (probably Gretney).
See http://www.isle-of-man.com/manxnotebook/famhist/mis/mn.htm
Ann also had a son Stanley born 1792 who also died in October 1795, possibly also smallpox.

Their second David was born 1796 in Douglas but I haven’t any further record of him, unless he is the David who married Sarah in the 1820’s or Eleanor Roberts in 1819 in Liverpool.

I have also been following the trail of Cretney’s off the Island and have also found the ‘David Clan’ or ‘Clans’ in Liverpool.
I have found your David C born 1825 (father David Beloved-mother Sarah ?) with brothers/sisters as follows
JAMES CRETNEY Christening: 27 JAN 1828 Saint Peter, Liverpool,
ISABELLA CRETNEY Christening: 11 MAY 1829 Saint Peter, Liverpool
SARAH JANE CRETNEY Christening: 12 SEP 1830 Saint Peter, Liverpool
SARAH ANN CRETNEY Christening: 17 JUL 1832 Saint Peter, Liverpool
ELLEN CRETNEY Christening: 12 NOV 1834 Saint Peter, Liverpool
I’ve no record of an Alexander.

The first Liverpool David I found was

DAVID CRETNEY Marriages: Spouse: ELEANOR ROBERTS
Marriage: 21 MAR 1819 Saint Peter Church Street, Liverpool
With no other information

Also DAVID CRETNEY married Year: 1842 Quarter: Sep Record Type: Marriages District: Liverpool County: Lancashire Volume: 20 Page: 312
possibly to a Mary ? with children
David C born abt May 1858 West Derby Lancs
Ann C born abt 1862
Mary C (widow) living with son & married daughter in 13 Roddlesworth St West Derby Lancs in 1881 census.

Also I presume your David again
DAVID CRETNEY Marriages: Spouse: MARY ANN NICHOLSON
Marriage: 24 NOV 1851 Saint Nicholas, Liverpool, Lancashire, England
They had a son DAVID CRETNEY Birth: 17 FEB 1859 Christening: 19 SEP 1859 Saint Peter, Liverpool,

I also found a DAVID C and Eleanor had a daughter Mary Eleanor Cretney Christening: 17 SEP 1883 St Peter's but cannot fit him into the jigsaw.

Regarding Alexander C
My first encounter is an Alexander CRETNEY Baptism: 2nd MARCH 1783 Mother Ann Cottier and brother of the Davids’.

The next ALEXANDER is found in Liverpool the 1881 census married to Elizabeth and born in Liverpool about 1855. He also had a son ALEXANDER born 1877 who died in WW1 age 43 in Oct 1917.
Alexander (1877) also had a son ALEXANDER born 1898 to Agnes Duncan. He is possible the Alex. recorded as steward on Ellis I records USA on ship ‘Adriatic’ in 1918 reported as unmarried.

I think that is enough ramblings for the time being. I hope you can confirm this information, add to it or correct it.

Regards Allan C