I have also record of Robert Quilliam as follows:

Robert Quilliam Father: Robert Quilliam ? Mother: Jane Key?
Christened: 07 Jul 1811 Marown?
Source: IGI - Not Verified Batch # C038221 Dates: 1622 -1849
Source #: 0106733 Type: Film Printout #: 6909411 Type: Film Sheet: 00

Catherine Cretney can be found on IGI as below:

Catharine Cretney Christening: 07 JAN 1810 Marown, Isle Of Man, England
Parents: Father: Robert Cretney Mother: Isabel Fargher
Source Information: Film Number: 442420 Page Number: 136 Reference number: 98478

Robert C & Isabel had other children as follows:

1. Robert Cretney Christening: 20 FEB 1791 Marown, Isle Of Man, England
2. William Cretney Christening: 29 OCT 1793 Marown, Isle Of Man, England
3. John Cretney Christening: 23 OCT 1796 Marown, Isle Of Man, England
4. Isabel Cretney Christening: 1799 Marown, Isle Of Man, England
5. Thomas Cretney Christening: 29 NOV 1801 Marown, Isle Of Man, England
6. James Cretney Christening: 06 MAR 1804 Marown, Isle Of Man, England
7. Joseph Cretney Christening: 19 APR 1807 Marown, Isle Of Man, England
8. Catharine Cretney Christening: 07 JAN 1810 Marown, Isle Of Man, England

I think the two Christine’s Cretney & Stephens are two different people as CC died in May 1843 while CS seemed to have children upto 1849 if the 1851 census is correct
RQ & CC had two children as follows:

CATHARINE QUILLIAM Christening: 08 MAR 1835 Marown, Isle Of Man, England
Parents: Father: ROBERT QUILLIAM Mother: CATHARINE CRETNEY
Source Information: Batch Number: 7006906 Sheet: 10 Source Call No.: 0538211 Type: Film
ISABELLA QUILLIAM Christening: 01 MAR 1840 Marown, Isle Of Man, England
Parents: Father: ROBERT QUILLIAM Mother: CATHARINE CRETNEY
Source Information: Batch Number: 7006906 Sheet: 10 Source Call No.: 0538211 Type: Film

I hope this helps in your deliberations.

Allan Cretney