Hi David F

Appears neither of those marriages are your James.
Had another look at what I had from 2013 , starting with Lace's Will that Donna Douglass posted for you , whereby Lace left legacies to sons John Thomas (now in America) James (now in Australia) , Edward (now in America), Joseph (now in America) and Benjamin of Ramsey. The full estate went to eldest son William Lace.
Had a look at 1841 census and found (ages rounded)
Lace 40
Esther 40
William 15
Catherine 15
John 13
Lace 10
James 7
Edward 5
Joseph 3
Benjamin 1
Margaret 6 mths

1851 census
Lace 51
Esther 51
William L 28
Catherine 26
Edward 16
Benjamin 11

James was not with his family , some may have been in America , can't find what happened to Lace Jnr , he did not die on the IOM , and not in will.
Margaret 6 mths in 1841 is strange , can't find her birth or any death to 1851 and not in will.
Do you have James on the 1851 census ?
Smiths Directory 1883 of Andreas has a Lace Radciffe Snr Grocer and Jnr Saddler , Snr could be your Lace Jnr , but as I mentioned , they did not stay on the Island.
I noted you have seen some of the original documents now on the IGI. Lace's birth 1800 and marriage is there , he signed his name in 1821 , Esther made her "Mark".
Also on the IGI is the incorrect death of your James 12/9/1836.
Census details came from Brian Lawsons Census Index , There are a lot of James Radciffes in 1841 , the ones around your James

James Radciffe age 7 Andreas, age 7 Braddan , age 6 , Andreas age 6 Arbory , age 6 Ramsey.

One thing for sure is that you have the right James Radcliffe , six of his ten children have Radcliffe family names - as Parker :

1858 Catherine
1860 William
1862 Edward
1864 James
1867 twins Esther & Benjamin

Cheers
David D