Death certs are notoriously inaccurate, because they are written by others who may have half-remembered birth place information, or made a guess.
You need information given by the people themselves, so if they said in the 1850 census that they were born in the IOM, that is far more likely to be correct.
Have you checked the other US censuses, some of which might give more details?

The IGI has an extracted (from the parish register) entry for the marriage of William KELLY & Mary Ann WILSON on 11 Apr. 1850 Braddan. Looking at the Braddan marriage record should be your next step for finding out both fathers' names and occupations, and addresses and other details.

The Braddan parish register copies are available on LDS film no. 0106188 [1849 - 1860], and this is available in the Manx Museum or for a small viewing fee through any of the LDS Family History Centres worldwide (see "Church of Jesus Christ of Latter-Day Saints" in the local phone book). You have sole use of the film for a month to view on their readers, and you may be able to find earlier family bapts & burials on the same film - once you know for certain who they were from the marriage record.

Sue